REPRODUCTION AND HABITAT USE
BY SAGE GROUSE (Centrocercus urophasianus)
IN A NORTHERN FRINGE POPULATION
A thesis
Submitted to the Faculty of Graduate Studies and Research
In Partial Fulfillment of the Requirements
For the Degree of
Master of Science
In Biology
University of Regina
by
Cameron L. Aldridge
Regina, Saskatchewan
June, 2000
© Cameron L. Aldridge 2000
ABSTRACT
Sage Grouse (Centrocercus urophasianus) are the largest of all North American
grouse and depend on sagebrush (Artemisia spp.) for food and protective cover. In
Canada, Sage Grouse are at the northern edge of the species’ range, occurring only in
southeastern Alberta and southwestern Saskatchewan. The Canadian population has
declined by 66 to 92% over the last 30 years. I used radiotelemetry to monitor Sage
Grouse survival and measure productivity in southeastern Alberta, and assess habitat use
at nesting and brood rearing locations. Annual male survival was low (31%), and female
survival may also be low (spring to fall; 56.5%) if overwinter mortalities are considered.
All females (n = 22) attempted to nest. Nest success (46.2%) and breeding success
(54.5%) were within the range found for more southerly populations (15 to 86% and 15
to 70%, respectively). Sage Grouse selected nest areas based on stands that had greater
amounts of tall sagebrush cover at a scale between 7.5 and 15 m in radius. Nests were
under the densest sagebrush present. Clutch size (7.8 eggs/nest) was within the normal
range for Sage Grouse (6.6 to 9.1), but at the high end of the spectrum. Fledging success
was comparable to that reported in other studies; however, chick survival to ≥ 50 days of
age (18%), was less than half of the required 35% survival for a stable or only slightly
declining population. Brood rearing sites were selected based on greater amounts of
taller sagebrush cover, but not forb content. High quality mesic areas containing 20 to
40% cover of lush forbs that chicks eat were limiting in southern Alberta (only 12%
cover available), despite spring precipitation, which was above average in both years of
my study. I developed a population model based on these estimated parameters. The
model predicts that by 2018, the Sage Grouse population in Alberta will fall to below 100
ii
birds and the Canadian population to ≤ 190 birds, which may not be sufficient to sustain a
viable population.
iii
ACKNOWLEDGEMENTS
My research would not have been possible without financial and/or logistical
contributions from the following organizations: Alberta Conservation Association,
Alberta Environmental Protection, Alberta Sport Recreation Parks & Wildlife
Foundation, Cactus Communications (Medicine Hat, AB), Canada Trust Friends of the
Environment Community Fund, Canadian Wildlife Foundation, Ducks Unlimited Canada
(North American Waterfowl Management Plan), Endangered Species Recovery Fund
(World Wildlife Fund Canada, Canadian Wildlife Service, and the Government of
Canada's Millennium Partnership Program), Esso Imperial Oil (J. Schacher, Manyberries,
AB), Mountain Equipment Co-op, Murray Chevrolet Oldsmobile Cadillac (Paul Murray,
Medicine Hat, AB), Nature Saskatchewan, Saskatchewan Environment and Resource
Management, Saskatchewan Stock Growers Association, Saskatchewan Wildlife
Federation, Nova/Trans Canada Pipelines Ltd., and the University of Regina.
I was partially supported throughout my research by a Macnaughton Conservation
Scholarship from the World Wildlife Fund of Canada, a Dennis Pattinson Memorial
Scholarship from the Saskatchewan Wildlife Federation, an Edgar A. Wahn Scholarship,
and Graduate Studies scholarships from the University of Regina.
I became interested in Sage Grouse in April 1997, when I worked for Alberta
Environmental Protection and performed Sage Grouse lek counts. I thank D. Hill, who
informed me about the position, and also opened my mind to biology with my first
exposure to field work in 1995. I thank D. Eslinger and R. Russell (both from Alberta
Environmental Protection) for giving me the opportunity to work with Sage Grouse in
1997. Over the course of my research, I had many helpful conversations with M.
iv
Schroeder (Washington Department of Fish and Wildlife), K. Lungle, D. Eslinger and J.
Taggart (all from Alberta Environmental Protection), J. Connelly (Idaho Fish and Game).
I also thank S. Brechtel (Alberta Environmental Protection), D. Prescott (Alberta
Conservation Association), K. Lungle, and I. Michaud (Alberta Conservation
Association), who provided me with a contract in 1997 to prepare the provincial status
report for Sage Grouse in Alberta.
I thank M. Brigham, T. Cobb, J. Foster, A. Gooliath, D. Kolybaba, M. Mclash, J
Morissette, D. Peltzer, R. Poulin, D. Schock, and D. Todd (University of Regina), C.
Braun (Colorado Division of Wildlife), J. Dionne, K. Grisley, and T. Wellicome
(University of Alberta), K. Kendell, I. Michaud, L. Takkats, and M. Wendlandt (Alberta
Conservation Association), R. Fisher (Valley Pet, Medicine Hat), M. Barrett (Alberta
Environmental Protection), S. Brechtel, D. Eslinger, K. Lungle, R. Russell and J. Taggart,
for assistance in the field and/or with various aspects of my research. I am grateful for
statistical assistance and advice from N. Kenkel (University of Manitoba). I thank J.
Medby and M. Friebel of the Department of Biology at the University of Regina for their
support and assistance. In particular, I was blessed with three individuals who worked
exceptionally hard in the field. For their efforts, I thank to C. Cullins ‘the Guard Cock’,
and ‘the Little Red Hens’, E. Urton, M. Watters, and T. Seida. T. Seida was with me
from the inception of my fieldwork and was invaluable in both years.
I am indebted to my best friend, J. Dionne, for her friendship, understanding, and
‘Crash and Burn’ support throughout the course of my research. I am especially thankful
to R. Poulin, for his interest in my research and for always having his door open for
discussions about my project. Many discussions with D. Gummer, and the various
v
members of ‘Bird’n Bat Lab’ over the course of my stay in Regina were also greatly
appreciated. I thank W. Harris (Saskatchewan Environment and Resource Management)
for providing data on Sage Grouse lek counts in Saskatchewan and for creating the
Canadian Sage Grouse range map.
I am appreciative of the many individuals and families who gave me permission
to work on their land throughout the course of my study. I especially thank C. Pearson
and family, R. Carry, N. Finstad, L. Finstad, M. Murray and family, the Kusler families,
the Heydlauff family, B. Biesterfeldt, T. Biesterfeldt, F. Gracey, the Nemiscam Grazing
Association, the Sage Creek Grazing Reserve, the Cressday Grazing Association, and the
Comrey Grazing Co-op, for allowing me access to their land. I thank R. Weeks for
generously sharing his flying expertise, and spending many early morning hours
searching for Sage Grouse from the skies above.
I also thank the late B. Bohnet, J. Dunsmore, G. and C. Peters, J. and L. Schacher,
A. and P. Girard, and W. Mckinnley for their genuine Manyberries hospitality. B.
Bohnet and J. Dunsmore were especially kind in providing me with accommodations at
the ‘Grouse House’. I also thank ‘The Fonz’ for keeping my field crew entertained
throughout our summers in Manyberries.
I thank my graduate supervisor, M. Brigham, for his guidance and support of my
research, and for believing in me as a biologist. He has fostered my development as both
a biologist and a human being, and I am grateful for his friendship. I am indebted to C.
Braun (Colorado Division of Wildlife), who was kind enough to take me under his wing
and furthered my development as a biologist. I thank him for all of his encouragement
and assistance with my research from the planning stages to the completion of my thesis.
vi
He has inspired my career as a biologist, and I am grateful for his friendship. S. Wilson
(University of Regina), W. Chapco (University of Regina), and C. Braun comprised my
thesis committee and provided advice and critical review of my study.
I am forever grateful to my parents, L. Aldridge and B. Aldridge for their support
and encouragement that has allowed me to pursue my career in biology, and for raising
me to appreciate the small things in life.
Finally, I thank the Sage Grouse, for allowing me to enter into their world and
attempt to understand their role within the prairie ecosystem. Through these birds, my
mind has been opened to many things and I have learned many more life lessons.
vii
TABLE OF CONTENTS
ABSTRACT........................................................................................................... ii
ACKNOWLEDGEMENTS ................................................................................ iv
TABLE OF CONTENTS .................................................................................. viii
LIST OF TABLES ............................................................................................... xi
LIST OF FIGURES ............................................................................................ xii
GENERAL INTRODUCTION ............................................................................ 1
CHAPTER 1.
STATUS OF SAGE GROUSE IN CANADA ........................ 3
INTRODUCTION .............................................................................................. 4
DISTRIBUTION................................................................................................. 8
PROTECTION.................................................................................................... 9
Federal ............................................................................................................ 9
British Columbia ............................................................................................. 9
Saskatchewan................................................................................................ 10
Alberta........................................................................................................... 10
POPULATION SIZE AND TRENDS .............................................................. 11
Between Year Trends .................................................................................... 12
Seasonal Lek Attendance .............................................................................. 17
HABITAT ......................................................................................................... 20
Leks............................................................................................................... 20
Nesting Areas ................................................................................................ 20
Brood Rearing............................................................................................... 21
Wintering Habitat ......................................................................................... 22
viii
GENERAL BIOLOGY..................................................................................... 23
Lek Behaviour ............................................................................................... 23
Nesting .......................................................................................................... 24
Non-breeding Season .................................................................................... 25
Diet ................................................................................................................ 26
Survival ......................................................................................................... 28
LIMITING FAC TORS ..................................................................................... 28
Agricultural Practices................................................................................... 29
Human Disturbance ...................................................................................... 30
Predation....................................................................................................... 30
Oil and Gas Exploration ............................................................................... 31
Roadways and Traffic ................................................................................... 32
Climate.......................................................................................................... 32
SPECIAL SIGNIFICANCE OF THE SPECIES .............................................. 33
EVALUATION................................................................................................. 34
CHAPTER 2.
NESTING AND REPRODUCTIVE ACTIVITIES ............ 35
INTRODUCTION ............................................................................................ 36
METHODS AND MATERIALS ...................................................................... 38
RESULTS ......................................................................................................... 41
Initiation of Incubation ................................................................................. 42
Clutch Size..................................................................................................... 42
Nesting Attempts........................................................................................... 43
Nest Success .................................................................................................. 43
Renesting Likelihood..................................................................................... 45
Breeding Success........................................................................................... 46
Fledging Success........................................................................................... 46
Chick Survival............................................................................................... 46
DISCUSSION ................................................................................................... 47
ix
CHAPTER 3.
NESTING AND BROOD HABITAT USE .......................... 58
INTRODUCTION ............................................................................................ 59
METHODS AND MATERIALS ...................................................................... 61
RESULTS ......................................................................................................... 65
Nests.............................................................................................................. 68
Nesting Habitat Characteristics by Nest Fate.............................................. 69
Nest Habitat .................................................................................................. 71
Brood Habitat ............................................................................................... 75
Line Transects............................................................................................... 77
DISCUSSION ................................................................................................... 79
CHAPTER 4.
MODELING POPULATION TRENDS .............................. 87
LITERATURE CITED ...................................................................................... 98
x
LIST OF TABLES
Table 3.1. Spearman rank correlations for vegetation variables measured at all Sage
Grouse nest, brood, and random locations combined ............................................... 67
Table 3.2. Vegetation characteristics at successful and unsuccessful nests .................... 70
Table 3.3. Discriminant Functions Analysis of vegetation characteristics at successful
and unsuccessful nests .............................................................................................. 72
Table 3.4. Vegetation characteristics at nests and random locations............................... 73
Table 3.5. Vegetation characteristics at brood use and random locations ....................... 76
Table 3.6. Sagebrush characteristics at use locations (nest and brood) and random
locations along line transects .................................................................................... 78
Table 4.1. Parameters used in population model ............................................................. 91
xi
LIST OF FIGURES
Figure 1.1. Current and known historic distribution of Sage Grouse. (Adapted from
Johnsgard 1983) .......................................................................................................... 5
Figure 1.2. Range of Sage Grouse in Canada .................................................................... 6
Figure 1.3. Population trends for Sage Grouse in Alberta and Saskatchewan
from 1968 to 1999..................................................................................................... 13
Figure 1.4. Estimated Sage Grouse population in Alberta from 1968 to 1969................ 16
Figure 1.5. Predicted lek attendance for male and female Sage Grouse (adapted from
Jenni and Hartzler 1978)........................................................................................... 18
Figure 1.6. Weekly attendance by male Sage Grouse on leks in Alberta
from 1996 to 1999..................................................................................................... 19
Figure 2.1. Clutch size for first nests and renesting attempts by Sage Grouse in
southeastern Alberta in 1998-99 ............................................................................... 43
Figure 2.2. Kaplan-Meier product limit estimate for nest survival in relation to date after
initiation, for Sage Grouse nests in southern Alberta in 1998 and 1999 .................. 44
Figure 2.3. Timing of nest failure for 14 unsuccessful Sage Grouse nests in southeastern
Alberta in 1998-99 .................................................................................................... 45
Figure 4.1. Maximum Sage Grouse lek counts in Alberta shown as a function of spring
(April - June) precipitation (mm) for each year ........................................................ 90
Figure 4.2. Actual Sage Grouse population size in Alberta based on lek counts for 1968
through 1999 shown with the predicted population superimposed .......................... 92
Figure 4.3. Actual Sage Grouse population in Alberta based on lek counts for 1968
through 1999 and predicted population from 2000 to 2030 ..................................... 93
xii
GENERAL INTRODUCTION
Sage Grouse (Centrocercus urophasianus) are strongly associated with sagebrush
(Artemisia spp.) habitat within the prairie ecozone. Population declines have been
reported throughout North America and range from 33 to 80% (Connelly and Braun
1997, Braun 1998). Declines have been most severe at the northern fringe of the species’
range, where the Canadian population has declined by at least 80% from historical levels
(Aldridge 1998).
The long-term decline in Sage Grouse populations across their range was
originally due to the loss of 2.5 million ha of sagebrush steppe habitat since the early
1900s (Patterson 1952, Eng and Schladweiler 1972, Braun 1995). Historically, Sage
Grouse occupied approximately 100,000 km2 within Alberta and Saskatchewan, but
today occupy only about 6,000 km2 (Aldridge 1998). With a reduction in range of
approximately 90% within the two provinces, this is the most severe range contraction
throughout the species’ range.
Many factors may be contributing to the Canadian Sage Grouse population
decline, including 1) reduced reproductive success, and 2) reduced survival of postfledged young and/or adults. Habitat degradation, fragmentation, disturbance, predation
pressure, reduced nest success, and climatic change all influence reproduction and
survival.
The overall objective of my study was to collect data on the population dynamics
and ecology of Sage Grouse in Canada; specifically the Alberta population, in an attempt
to identify, from a proximate perspective, why the Canadian Sage Grouse population is
declining. In 1998 and 1999 I performed spring lek counts to monitor population trends
1
and estimated survival of adults through the use of radiotelemetry. I followed females to
their nesting sites and monitored measures of reproductive effort and reproductive
success. Females that successfully hatched their chicks were followed so that I could
estimate chick survival, and thus, overall recruitment. I used these data to assess the
status of the Alberta and Canadian populations, and used the various population
parameters to predict future population trends and compare measures of productivity to
other stable and declining populations in the United States.
I measured vegetation characteristic at nest locations, and in habitats used for
brood rearing. By assessing habitat use by Sage Grouse, I was able to assess selection,
and compare the quality of available habitat to that used by other populations. This
allowed me to make inferences about how habitat quality might affect survival and
productivity, and thus, population trends.
I have chosen to structure my thesis as series of individual Chapters, each of
which is written as a manuscript that will be submitted to a peer reviewed scientific
journal. Thus, there is some repetition of data and results between individual Chapters.
The first Chapter, (Status of Sage Grouse in Canada), is written for submission to the
Canadian Field Naturalist. Thus, sections within this Chapter are arranged based on the
format for a species status and distribution manuscript for that journal. Chapter Two,
Nesting and Reproductive Activities, and Three, Nesting and Brood Habitat Use, are each
written in a style for publication in an ornithology journal. The final Chapter, Modeling
Population Trends, ties together all the population parameters that I measured, and
explains the population model that I have developed to predict the Alberta/Canadian Sage
Grouse population.
2
CHAPTER 1.
STATUS OF SAGE GROUSE IN CANADA
3
INTRODUCTION
Sage Grouse (Centrocercus urophasianus) are strongly associated with sagebrush
(Artemisia spp.) habitats throughout the Great Plains and Intermountain West.
Historically, they occurred in British Columbia, Alberta, Saskatchewan and at least 16
U.S. states, but they have been extirpated from British Columbia and five states (Braun
1998, Schroeder et al. 1999) (Fig. 1.1). The long-term decline seen in Sage Grouse
populations across their range was originally due to the direct loss of the sagebrush
steppe associated with grassland habitats. This habitat has been reduced by more than
50% (2.5 million ha) since the early 1900s (Patterson 1952, Eng and Schladweiler 1972,
Braun 1995) (Fig. 1.1). Of the two subspecies of Sage Grouse, the eastern race (C. u.
urophasianus) is found at the northern edge of its range in extreme southeastern Alberta
and southwestern Saskatchewan (Fig. 1.1).
Both Alberta and Saskatchewan still support Sage Grouse populations (Fig. 1.2),
yet springtime lek counts indicate the Canadian population has decreased by
approximately 80% since the mid 1980s, and was between 549 to 813 individuals in 1997
(Aldridge 1998). As a consequence of the decline, Alberta closed the Sage Grouse
hunting season in 1996 for the first time since 1967 (Aldridge 1998). Sage Grouse have
not been hunted in Saskatchewan since 1938 (Kerwin 1971).
In 1997, the Committee on the Status of Endangered Wildlife In Canada
(COSEWIC) listed Sage Grouse as a Threatened species. This listing was upgraded to
Endangered in 1998, reflecting the imminent threat of extinction of Sage Grouse in
Canada (Hyslop 1998).
4
Figure 1.1. Current and known historic distribution of Sage Grouse. ‘E’ represents the
eastern subspecies (C. u urophasianus) and ‘W’ represents the western subspecies
(C. u. phaios). The current distribution is not continuous and is more fragmented
than indicated. (Adapted from Johnsgard 1983).
5
Figure 1.2. Range of Sage Grouse in Canada. Historical range is based on anecdotal sightings of birds prior to the 1960s. The
present (1997) range is based on the locations of known active leks in 1997. The 1987 range limits are shown to illustrate the
range contraction. (Map produced by W. C. Harris).
6
Sage Grouse are found almost exclusively within the North American range of
sagebrush (Artemisia spp.), and are associated with sagebrush habitats throughout the
year (Patterson 1952, Eng and Schladweiler 1972, Wallestad 1975, Braun et al. 1977,
Crawford and Lutz 1985, Swenson et al. 1987, Braun 1995). This is also the case in
Canada, where Sage Grouse are found within the range of sagebrush in the semi-arid
mixed-grass prairie. Silver sage (A. cana) is the main species of sagebrush on the
Canadian prairies and is most frequently associated with grasses such as june grass
(Koeleria macrantha), blue grama (Bouteloua gracilis), speargrass (Stipa comata), and
western wheatgrass (Agropyron smithii). Pasture sage (Artemisia frigida) is the main
forb (Aldridge 1998). Mean annual precipitation within the Alberta range is about 332
mm, and mean temperatures for July and January average 19.5 and -11.7EC, respectively
(Onefour Research Station, Environment Canada).
Although Sage Grouse have a close association with sagebrush habitats, specific
habitat requirements vary throughout the year. Areas used by Sage Grouse must contain
suitable habitat which satisfies requirements for strutting grounds (leks), nesting areas,
feeding and loafing sites, brood rearing sites, and wintering areas (Klebenow 1969, Eng
and Schladweiler 1972, Wallestad and Pyrah 1974, Beck 1977).
The purpose of this Chapter is to evaluate the status of the Sage Grouse in
Canada, discussing the species distribution, biology, habitat requirements, and potential
factors that might be affecting the population. I will also discuss long term and present
population trends, addressing the population decline, range contraction, and relevant
research.
7
DISTRIBUTION
Sage Grouse have been extirpated from at least five U. S. states and British
Columbia (Braun 1998, Schroeder et al. 1999) (Fig. 1.1). Throughout their range, Sage
Grouse have declined by 45-80% since the 1950s (Braun 1998). The long-term decline
was originally due to the direct loss of sagebrush steppe associated with grassland
habitats (Patterson 1952; Eng and Schladweiler 1972; Braun 1995, 1998; Schroeder et al.
1999). Sage Grouse presently inhabit about 50% of the area they once occupied in
Oregon (Crawford and Lutz 1985) and Colorado (Braun 1995) at the turn of the century.
Range contractions of similar magnitude have occurred elsewhere in the species’ range
(Patterson 1952, Eng and Schladweiler 1972, Wallestad 1975, Braun et al. 1977,
Crawford and Lutz 1985, Swenson et al. 1987, Braun 1995). The current distribution of
Sage Grouse is fragmented (Patterson 1952, Eng and Schladweiler 1972, Hupp and Braun
1991, Braun 1995) (Fig. 1.1).
The eastern subspecies (C. u. urophasianus) is at the northern edge of its range in
extreme southeastern Alberta and southwestern Saskatchewan (Fig. 1.1). Historically,
Sage Grouse occupied approximately 100,000 km2 within the two prairie provinces, but
today, occupy only about 6,000 km2 (Fig. 1.2). The range contraction within Canada is
primarily attributed to habitat loss.
The western subspecies (C. u. phaios) occurs in smaller numbers from eastern
Washington to southeastern Oregon (Fig. 1.1). Historically, this subspecies extended into
the southern Okanagan and Similkameen valleys of British Columbia, but was considered
extirpated by 1918 (Cannings et al. 1987).
8
A smaller-bodied Sage Grouse that is behaviourally, morphologically and
genetically different from the larger bodied birds, has been proposed as a distinct species;
the Gunnison Sage Grouse (C. minimus; Kahn et al. 1999, Oyler-McCance et al. 1999,
Schroeder et al. 1999). These birds are found from southwestern Colorado into
southeastern Utah, and were likely geographically isolated in the Pleistocene (C. E.
Braun, pers. commun.).
PROTECTION
Federal
Sage Grouse were hunted in Saskatchewan (prior to 1938) and in Alberta (prior to
1996), but are no longer hunted in Canada. There is some indication that Sage Grouse
populations can be hunted with minimal effects on population numbers (Braun 1984,
Braun and Beck 1985), however, hunting small populations in fragmented areas may
have significant implications.
After initially being listed as Threatened by COSEWIC in 1997, Sage Grouse
(prairie population) were upgraded to Endangered status in 1998 (Hyslop 1998).
However, until the new Federal Species At Risk Act (SARA) is passed, the species is
afforded little Federal protection. Thus, at this time, protection of Sage Grouse is limited
to that afforded by Provincial regulations.
British Columbia
Sage Grouse have not been observed in British Columbia since the 1960s
(Cannings et al. 1987), and the population is considered Extirpated (Hyslop 1998).
9
Saskatchewan
In Saskatchewan, Sage Grouse were listed as a Threatened species in 1987, based
on a declining population and a reduction in range. In 1999, they were upgraded to
Endangered. Now that Sage Grouse are on the Provincial Endangered species list, their
habitat is protected under The Wildlife Habitat Protection Act, which means that lands
containing Sage Grouse habitat can not be sold or have their native vegetation cultivated.
In 1994, Saskatchewan implemented restrictions that limited development and
disturbance at Sage Grouse lek sites. The Saskatchewan Wildlife Act was amended in
1997 to list and protect wild species at risk. With the 1999 Saskatchewan classification
of Endangered, Sage Grouse are now protected under the Saskatchewan Wildlife Act.
These regulations provide protection of Sage Grouse, their nests, and leks sites. No
developments within 500 m of leks are permitted and no construction activities within
1000 m of leks are allowed between 15 March to 15 May.
Alberta
In 1991, Sage Grouse in Alberta were given a “Yellow” listing, meaning they
were considered a species of concern due to their naturally low populations, their limited
distribution, and the limited available habitat (Anonymous 1991). They were
subsequently added to the “Blue” list of species that may be at risk in 1996 (Anonymous
1996). This designation was assigned due to the species’ limited distribution, declining
population numbers, and specific habitat requirements. Despite this, Sage Grouse are
still considered a game bird in Alberta. The Alberta Endangered Species Conservation
Committee has recommended Sage Grouse be listed as Endangered under the Alberta
10
Wildlife Regulations (K. J. Lungle, pers. commun.). By virtue of being a non-hunted
species, they are afforded limited protection. Once they are listed provincially as an
Endangered species, more rigid protection will be available to protect against the capture,
killing or harming of individuals or their nests.
Sage Grouse habitat is not currently protected within the Province of Alberta,
although there is the potential to enforce and protect habitat under other provincial
legislation. Listing Sage Grouse as a provincially Endangered species will put the
Endangered Species Conservation Committee in charge of the species’ recovery. It will
also afford significantly higher enforcement penalties and provide greater opportunity to
protect Sage Grouse and their habitat.
Alberta Environmental Protection has developed recommendations and land use
guidelines, which propose to limit activities surrounding Sage Grouse leks throughout the
year. From 16 June to 29 February, seismic activities, surveying, and monitoring would
all be prohibited within 100 m of leks, and from 1 March to 15 June, these activities
would be prohibited within 500 m. Permanent developments would be prohibited within
1000 m of leks, regardless of the time of year. However, these recommendations can not
currently be legally enforced and are only guidelines.
POPULATION SIZE AND TRENDS
The most cost effective and time efficient method to estimate Sage Grouse
population size is through lek counts. Lek counts involve counting the number of male
Sage Grouse displaying on a strutting ground (lek) during the spring mating season. The
maximum number of males observed at each lek is then used as an index of population
11
status (Beck and Braun 1980, Emmons and Braun 1984). Lek counts are used to make
population estimates and as indicators of population trends for many lekking species,
including all Sage Grouse populations, even though there is no direct evidence of a
relationship between attendance of males at leks and population size.
Between Year Trends
Sage Grouse lek counts within Alberta and Saskatchewan have been performed
independently, and for this reason, I discuss population trends separately (Fig.1.3). In
Alberta, surveys have been performed on average, every two years since 1968, although
gaps as long as five years have occurred (Fig. 1.3). During 1968/69, and in the early
1980s, Sage Grouse numbers peaked, and approached 600 males counted on about 20
leks, with an average of greater than 25 males/lek (Fig. 1.3). In Saskatchewan, the first
surveys were performed in 1987 and 1988. They resulted in counts of nearly 600 males
on about 30 active leks, with an average of about 20 males/lek (Fig. 1.3). Since surveys
began, there has been a general decline in population numbers. In 1997, there were only
8 and 10 active leks, supporting 122 and 61 males in Alberta and Saskatchewan,
respectively (Fig. 1.3).
The most intensive lek counts have been conducted over the last two years in both
provinces. Maximum lek counts in Alberta resulted in a total count of 147 males on 8
active leks in 1998, and 140 males on 8 leks in 1999 (Fig. 1.3). A similar trend occurred
in Saskatchewan, where a maximum of 144 males were counted on 12 active leks in 1998
and 131 males on 10 leks in 1999 (W. C. Harris, pers. commun.) (Fig. 1.3). I estimate the
12
Number of Males
Active Leks
Males/Lek
800
Alberta
Saskatchewan
600
400
200
0
1965
35
30
25
20
15
10
5
0
1965
35
30
25
20
15
10
5
0
1965
1970
1975
1980
1985
1990
1995
Alberta
Saskatchewan
1970
1975
1980
1985
1990
1995
Alberta
Saskatchewan
1970
1975
1980
1985
1990
1995
Year
Figure 1.3. Population trends for Sage Grouse in Alberta and Saskatchewan from 1968 to
1999 shown as the number of males, number of males per lek, and number of
active leks. Years when sampling efforts consisted of less than eight leks
surveyed are not included. Lines are shown to illustrate trends between years
with consecutive lek counts.
13
1999 Canadian spring population was between 813 and 1204 individuals. These
estimates are based on spring lek counts of males, and the low estimate assumes a spring
sex ratio of two females for every male; whereas the high estimate assumes the same sex
ratio, but also takes into account the potential that only 90% of all leks are located and
that only 75% of males attend leks at any given time.
In 1987, counts were performed in both provinces, and a total of 915 males (400
in Alberta, 515 in Saskatchewan) was counted at 34 active leks (Fig. 1.3). This gives an
estimated 1987 spring population of between 2745 and 4067 individuals. Based on 1999
estimates, the population has declined by as much as 80% since 1987. However, the
1987 total likely does not represent historic population levels, considering that Alberta
counts in 1968 and 1981 in Alberta totaled 613 and 524 males respectively, and counts
were also greater in Saskatchewan in 1988 (677 males). If these numbers are used to
estimate a historic (≈ 1968) spring Sage Grouse population for the Canadian prairies
(only covering the current range), the population would have been between 3870 and
5733 individuals. This represents a potential decline of 86% over the last 3 decades,
which may be underestimated, considering the historic range of Sage Grouse was likely
much greater (Fig. 1.2). Search efforts have been much greater over the last six years in
both provinces, and likely resulted in higher count totals for the surveyed leks. Thus, the
population decline may be even greater than lek counts indicate.
While it is evident the Canadian Sage Grouse population has declined, the exact
rate of decline is difficult to ascertain, due to inconsistent sampling efforts (Madsen
1995b; Aldridge 1997, 1998). It is also difficult to determine whether in some years, leks
that apparently contained no birds, were simply not checked, or could not be located and
14
surveyed (Aldridge 1998). The assessment of trends is further complicated by the fact
that Sage Grouse populations appear to cycle every 7 to 10 years (Fig. 1.3) (Patterson
1952, Rich 1985, Aldridge 1998, Braun 1998, Hyslop 1998). Regardless of the decline, it
has been suggested that a minimum population of 500 (Franklin 1980, Lande 1988) and
possibly even 5000 (Braun 1995, Anonymous 1997) individuals may be required to
maintain sufficient genetic diversity to sustain a viable population. This number may be
even higher for Sage Grouse, considering that only 10-15% of males actually breed in
any given year (Anonymous 1997; C. E. Braun, pers. commun.; Chapter 4).
Using long-term lek count data for Alberta (Aldridge 1998, Alberta
Environmental Protection Natural Resource Service), I estimated the population for each
year from 1968 to 1999 (Fig. 1.4) using the assumptions previously discussed. In the late
1960s, the Alberta population was between 1839 and 2724 birds. At its lowest levels in
1994, I estimate the population was between 210 and 311 individuals. In 1999, the
population consisted of between 420 and 622 individuals. Thus, the Alberta population
has declined from 1968 to 1999 by between 66 and 92%. This decline could be even
greater, considering that as little as 5 years ago, Sage Grouse were known to exist outside
of their current range, but these areas were not surveyed in past lek counts.
Similar declines are seen in terms of lek abandonment in both Alberta and
Saskatchewan. There were at least 21 active leks in Alberta in the late 1960s, and 31
active leks in 1988 in Saskatchewan (Fig. 1.3); 62 and 67% of leks have been abandoned
in each province, respectively. The mean number of males per lek has also decreased in
both provinces. Alberta averaged 29.2 males per lek in 1968, while Saskatchewan
15
averaged 21.8 in 1988. By 1994, these numbers had decreased by 80% in Alberta and
64% in Saskatchewan to 5.8 and 7.8 males per lek, respectively.
18 % Chick Survival
Number of Birds
3000
Low Estimate
High Estimate
2500
2000
1500
1000
500
0
1965
1970
1975
1980
1985
1990
1995
2000
Year
Figure 1.4. Estimated Sage Grouse population in Alberta from 1968 to 1969. Low
estimates are based on a spring sex ratio of two females for every male. High
estimates assume the same sex ratio; that only 90% of all leks are located; and
that only 75% of males attend leks at any given time. Years when sampling
efforts consisted of less than eight surveyed leks are not included. Lines are
shown to illustrate trends between years with consecutive lek counts.
Despite the overall decrease in population numbers, counts of males have
remained relatively stable over the last six years in both provinces (Figs. 1.3, 1.4).
During this period, 1995 was the last year that Sage Grouse were hunted in Alberta, even
though the season was short (1 week long) and the estimated harvest, based on hunter
surveys, was below 10% (K. J. Lungle, pers. commun.). However, over the last six years,
more rigorous lek counts have been performed each year in both provinces, and a
concerted effort has been made to locate all leks (Aldridge 1998; W. C. Harris, pers.
commun.). This increased effort may mask a continuing population decline.
16
Even though counts of males on leks over the last six years have remained
relatively stable, the number of active leks in Canada decreased from 22 in 1994 to 18 in
1999 (Fig. 1.3). Over the same time period, the mean number of males counted per lek
has more than doubled from 6.7 males/lek in 1994 to 15.1 in 1999 (Fig. 1.3). It is
common for smaller subsidiary or satellite leks to be abandoned during population lows
(Dalke et al. 1963), but the attendance at main lek complexes has increased over this time
period. This suggests that changes in habitat quality or fragmentation have been
occurring, making smaller leks less desirable and forcing birds to move to leks in more
suitable habitat.
Seasonal Lek Attendance
Adult males begin returning to leks once they are clear of snow. This typically
occurs in mid-March in Alberta (C. L. Aldridge unpubl. data). Yearling males do not
attend leks until after the peak of breeding activities. Breeding occurs over a one to two
week period, as indicated by the peak in female attendance at leks; late March - early
April in California (Bradbury et al. 1989), mid-April in Colorado (Petersen 1980), early
April in Idaho (Autenrieth 1981), mid- to late April in Montana (Wallestad 1975, Jenni
and Hartzler 1978), late April in Washington (Schroeder 1997) and in early April in
Alberta (Chapter 2). This peak typically occurs two to three weeks after peak hen
attendance/breeding at leks (Jenni and Hartzler 1978) (Fig. 1.5). This figure is based on
attendance of Sage Grouse at leks from three different studies (Dalke et al. 1963, Eng
1963, Jenni and Hartzler 1978). Attendance is shown as the percent of the seasonal
maximum attendance for both males and females. All three studies found that only about
17
50% of male Sage Grouse attend leks prior to the peak in female attendance (breeding)
(Fig. 1.5). Radiotelemetry studies indicate that yearling males do not attend leks until
two to three weeks after the peak of female attendance (Jenni and Hartzler 1978,
Emmons and Braun 1984).
Typical Lek Attendance
MALES
FEMALES
Attendance in Percent of
Seasonal Maximum
120
100
80
60
40
20
0
35
25
Days Before
15
5
- +5
Day of Peak
Hen Attendance
15
25
35
45
Days After
Figure 1.5. Predicted lek attendance for male and female Sage Grouse (adapted from
Jenni and Hartzler 1978). In Canada, peak female attendance usually occurs in
the first week of April.
The attendance of male Sage Grouse at leks in Alberta within each breeding
season varied slightly from 1996 to 1999 (Fig. 1.6). Lek counts were divided into 4
seven-day periods, spanning the three 10-day periods recommended by Jenni and
Hartzler (1978) and Beck and Braun (1980). The third week of counts occurred during
the last week of April, when the peak in male attendance at leks should occur (Aldridge
1998) due to the arrival of yearling males (Dalke et al. 1963, Eng 1963, Jenni and
18
Hartzler 1978). I obtained a maximum count for each lek during each of the four-week
periods and used a two-way ANOVA to evaluate Week*Year interaction (F9, 100 = 0.02, P
> 0.10), Week (F3, 100 = 0.11, P > 0.10), and Year effects (F3, 100 = 1.24, P > 0.10). The
lack of significant differences in the attendance of males at leks over each year as a
function of week suggests that yearling Sage Grouse are under represented in the
Canadian population, and that low recruitment may be contributing to the population
Number of Males/Lek
decline.
20
18
16
14
12
10
8
6
4
2
0
1996
I
1997
II
1998
III
1999
IV
Week
Figure 1.6. Weekly attendance by male Sage Grouse on leks in Alberta from 1996 to
1999. Week III represents the time when peak male attendance historically
occurred in Alberta and Saskatchewan.
19
HABITAT
Leks
Leks (strutting grounds) are sites where displaying males congregate and are
highly visible to females. They range in size from 0.04 to 4 ha and are traditional, with
some known to remain active for upwards of 100 years (Dalke et al. 1963). Leks are
typically in flat, open areas, such as dried mud flats or valley bottoms (Scott 1944,
Patterson 1952, Dalke et al. 1963, Peterson 1970). They are often slightly lower than
surrounding areas, and frequently located near standing water in Alberta (Aldridge 1998;
C. L. Aldridge, pers. observ.). Patterson (1952) reported that some leks were located on
slight knolls and ridges. Leks themselves are in areas with little vegetation but are
typically surrounded by sagebrush flats that are important as feeding and roosting sites
(Scott 1944, Patterson 1952, Peterson 1970, Clark and Dube 1984). Spring daytime
roosting sites of males have 20-50% sagebrush canopy cover and consist of plants that
are < 30 cm tall (Wallestad and Schladweiler 1974, Wallestad 1975).
Nesting Areas
Nesting habitat is largely associated with sagebrush flats surrounding strutting
grounds. Martin (1970) found that 80% of all nests were within 3.2 km of leks, and
similarly, Wallestad and Pyrah (1974) found that 68% of all nests occurred within 2.5 km
of a lek. Despite the apparent association of nests with leks, Wakkinen et al. (1992)
found that nest distribution with respect to leks was random, even though 92% of nests in
their southeastern Idaho study area occurred within 3 km of a lek. Nests are almost
exclusively placed under sagebrush plants (Patterson 1952, Klebenow 1969, Wallestad
20
and Pyrah 1974, Braun et al. 1977, Gates 1985) although in some areas Sage Grouse nest
under other shrub species which provide dense canopy cover (Connelly et al. 1991,
Sveum et al. 1998b). Connelly et al. (1991) found that yearling females placed their nests
under sagebrush more often than adults (95% vs. 79%). Sagebrush used for nesting has a
dense canopy cover (20-50%) and taller plants are preferred, ranging from 17 to 79 cm in
height (Patterson 1952, Klebenow 1969, Wallestad and Pyrah 1974, Schroeder et al.
1999).
Brood Rearing
Broods tend to shift from sagebrush uplands early in the brood rearing period, to
more mesic sites later in the summer (Patterson 1952, Dunn and Braun 1986b). In early
summer, broods concentrate in areas with sparse sagebrush that are more open and moist,
due to the presence of important foods such as succulent forbs (Patterson 1952,
Klebenow 1969, Wallestad 1971, Drut et al. 1994a). This shift is usually a result of the
desiccation of forbs in sagebrush uplands later in the summer (Dunn and Braun 1986b),
and reflects the importance of forbs in the diet of juveniles (Martin 1970, Peterson 1970,
Klott and Lindzey 1990, Fischer et al. 1993, Drut et al. 1994b, Pyle and Crawford 1996,
Sveum et al. 1998a). Females and broods searching for forbs move further into moist
areas (wetlands and wet meadows), away from sagebrush flats, and are often found near
open water (Patterson 1952, Klebenow 1969, Wallestad 1971, Autenrieth 1981, Drut et
al. 1994a). Habitat shifts occur when chicks are approximately seven weeks of age as
broods move to more mesic sites with lush forbs (Martin 1970, Drut et al. 1994b, Sveum
21
et al. 1998a). This shift coincides with dietary changes (Martin 1970, Peterson 1970,
Drut et al. 1994a).
Birds return to areas with dense sagebrush in late summer and fall before moving
to wintering grounds (Patterson 1952, Wallestad 1971, Drut et al. 1994b, Dunn and
Braun 1986a). Dunn and Braun (1986b) found that females and juveniles selected habitat
that was more homogeneous in terms of shrub size and density, and had the greatest
horizontal and vertical vegetational cover.
Wintering Habitat
During winter, sagebrush is extremely important as it makes up nearly 100% of
the diet of Sage Grouse, and provides cover during inclement weather (Patterson 1952;
Wallestad 1975; Johnsgard 1973, 1983; Remington and Braun 1985). In winter, birds are
usually found at lower elevations such as drainage basins (Patterson 1952, Hupp and
Braun 1989b), where sagebrush is tall enough to remain above the snow (Eng and
Schladweiler 1972, Johnsgard 1973). Southwest-facing slopes with a gradient of less
than 5% are important wintering areas, as they are wind-swept and relatively snow free,
leaving sagebrush exposed (Eng and Schladweiler 1972, Beck 1977). Eng and
Schladweiler (1972) found that 82% of all Sage Grouse winter locations were in
sagebrush stands with > 20% canopy cover, although sagebrush cover in winter habitat
ranges from 6 to 43% (Schroeder et al. 1999).
22
GENERAL BIOLOGY
Sage Grouse are the largest of all North American grouse (Johnsgard 1973, Beck
and Braun 1978). This species exhibits extreme sexual dimorphism, with females
averaging 48 to 58 cm in length and males 65 to 75 cm in length, and weighing about
1100 and 2400 g, respectively (Nelson and Martin 1953, Johnsgard 1973, 1983). Body
mass fluctuates throughout the year, with the largest gain taking place from January to
March (Beck and Braun 1978). Maximum mass is attained during the breeding season
(April to May) (Patterson 1952, Beck and Braun 1978, Hupp and Braun 1991). Beck and
Braun (1978) suggest that overwinter mass gain is necessary to meet the energy demands
of breeding, rather than for overwinter survival. Breeding mass for adult male Sage
Grouse in Alberta averages 3122 g (n = 48, C. L. Aldridge, unpubl. data). This is heavier
than most southern populations (3190 g in Colorado, Beck and Braun 1978; 2450 g in
Eastern Idaho, Dalke et al. 1963; 2900 g in central Montana, Eng 1963; 2700 g in
Wyoming, Patterson 1952). The greater mass of Sage Grouse at the northern edge of the
species’ range may be an adaptation to the more extreme weather conditions.
Lek Behaviour
Males begin returning to leks in late winter. They establish territories on leks and
display as soon as the snow begins to disappear. Older males arrive first, and obtain the
most central territories (Patterson 1952). After the period of peak female attendance,
yearling males begin to visit leks (Fig. 1.5). If a yearling male manages to obtain a
territory, he is usually displaced to the lek periphery. Each female attends a lek for a
23
period of 2 to 3 days and mates only once (Gibson and Bradbury 1986), typically with
one of the dominant males (Gibson 1996).
Males attend and display at leks at both dusk and dawn, but activity peaks during
the hour surrounding sunrise (Patterson 1952, Johnsgard 1983). The male display is used
both to attract females and defend a territory from other males (Patterson 1952, Johnsgard
1983). The display itself consists of a series of “struts,” in which the male fans his tail
feathers, inflates his esophageal air sacs and puffs out his white chest feathers while
displaying his olive green-yellow gular sacs. Males lift their drawn wings and produce a
brushing sound, and finish with a characteristic “plopping” sound that is produced as air
is released from the esophageal sacs (Patterson 1952, Johnsgard 1983). Displays are
most intense when females are present (C. L. Aldridge, pers. observ.). Both males and
females tend to return to the same strutting ground each year (Dalke et al. 1963, Eng
1963, Emmons and Braun 1984, Berry and Eng 1985, Fischer et al. 1993), and > 50% of
yearlings return to the strutting ground at which they were conceived (Dunn and Braun
1985). Inter-lek movements by adult males during the breeding season are uncommon
(Dalke et al. 1963, Wallestad and Schladweiler 1974). Juvenile males and females
sometimes move between leks (Emmons and Braun 1984).
Nesting
After mating, females move to nesting areas, which are typically close to the
previous years’ nest sites (Patterson 1952, Fischer et al. 1993, C. L. Aldridge, unpubl.
data). Egg laying is initiated within a few days of mating (Patterson 1952) and
approximately 1.3 days elapse between the laying of successive eggs (Patterson 1952,
24
Schroeder et al. 1999). Incubation lasts about 27 days (range 25 to 29; Schroeder et al.
1999, Chapter 2). Average clutch size is 7 to 9 eggs (Schroeder et al. 1999), and in
Alberta, peak hatching occurs in the first week of June (Chapter 2, Clewes 1968).
Wallestad and Pyrah (1974) found that nesting success of adult females was greater than
yearlings (77% vs. 44%). In Idaho, Connelly et al. (1993) found that 78% of all adult
females and only 55% of yearlings initiated a nest, yet 52% of both age groups produced
a clutch. Schroeder (1997) found that 100% of females attempted to nest with no agerelated effects on nest success.
In Alberta, mean brood size from 1967 to 1976 decreased from 4.4 to 3 chicks per
female (both n = 20; Windberg 1976). In 1985, brood size was 3.4 (n = 29; Banasch
1985). Crawford and Lutz (1985) reported similar trends in Oregon, with brood sizes
decreasing from 4.5 chicks per female in the late 1950s, to 3.3 in the early 1980s. They
also reported the percentage of adults with broods declined from a maximum of 55% to
only 9% over the same time period. Limited brood surveys conducted in Alberta in 1995
suggested that only 21% of females (n = 19) had broods, with an average of 1.5 chicks
per brood (n = 4; Madsen 1995a). Unfortunately, it is difficult to compare productivity
between these studies due to differences in sampling techniques.
Non-breeding Season
In late summer and fall, Sage Grouse gather in sexually segregated flocks,
although some flocks contain females and immature males (Eng and Schladweiler 1972,
Beck 1977). Movements to wintering grounds begin at this time (September to
November) and may last until December (Connelly et al. 1988). Distances moved from
25
breeding to wintering ranges averaged 7.9 to 11.6 km for Sage Grouse in northern
Colorado (Beck 1977), 28 to 30 km in Colorado (Schoenberg 1982), and 11.3 km for
adults in southeastern Idaho (Connelly et al. 1988). However, one-way migrations of 80
km are not uncommon (Patterson 1952, Dalke et al. 1963, Connelly et al. 1988) and
distances of 160 km have been reported (Patterson 1952). Movements of juveniles in
Colorado are sporadic and tend to follow corridors of sagebrush (Dunn and Braun
1986a). Similarly, fall movements by adults in Idaho were found to be slow and
meandering (Connelly et al. 1988). The longest reported migration movements are
usually of birds moving to lower elevations (see Patterson 1952, Connelly et al. 1988).
In Montana, Sage Grouse populations are considered non-migratory, with
minimal movements occurring between winter and summer ranges due to the overlap of
habitats (Eng and Schladweiler 1972). Wintering grounds in southeastern Idaho also
overlap with spring and summer ranges (Connelly et al. 1988). Sage Grouse in Canada
are also considered to be non-migratory, as winter ranges overlap with spring and
summer ranges (Aldridge 1998; C. L. Aldridge, unpubl. data). Beck (1977) found that
wintering areas composed only 7% of the sagebrush habitat, suggesting that winter
habitat may be the most limited resource (Patterson 1952, Eng and Schladweiler 1972,
Beck 1977, Remington and Braun 1985).
Diet
Sage Grouse lack a muscular gizzard necessary for grinding seeds or other hard
materials (Patterson 1952, Remington and Braun 1985). Consequently, their diet is
limited to soft vegetation such as sagebrush leaves and lush forbs and insects. Sagebrush
26
constitutes 62% of the overall diet (Wallestad et al. 1975) and makes up 100% of the diet
in winter (Patterson 1952).
All Sage Grouse include some lush forbs in their summer diet, but forbs appear to
be particularly important to pre-laying females (20 to 50% of diet) (Barnett and Crawford
1994). Forbs are also important to juveniles, making up 75% of the diet of Sage Grouse
< 12 weeks of age (Peterson 1970). The majority of forbs consumed are leaves and
flower buds of common dandelion (Taraxacum officinale), common salsify (Tragopogon
dubius), and prickly lettuce (Lactuca serriola) (Klebenow and Gray 1968, Peterson 1970,
Wallestad et al. 1975). In Canada, vegetation consumed by Sage Grouse includes sweet
clover (Melilotus spp.) alfalfa (Medicago sativa), pasture sage (A. frigida), common
salsify, and silver sagebrush (Kerwin 1971).
Insects are also an important component of the diet of juveniles (Patterson 1952,
Klebenow and Gray 1968, Peterson 1970, Johnson and Boyce 1990, Drut et al. 1994b).
Grasshoppers (Orthoptera), beetles (Coleoptera), and ants (Formicidae) have all been
found in the diets of juveniles, as well as adults (Patterson 1952, Kerwin 1971, Wallestad
et al. 1975). In feeding trials with captive chicks, Johnson and Boyce (1990) found that
increasing amounts of insects in the diet increased both growth and survival. In the wild,
insects may make up as much as 60% of the diet of chicks less than one week old
(Peterson 1970). A decrease in the consumption of insect matter and changes in plant
food sources occur in broods at about 7 weeks of age, which has been linked to a shift in
habitat use (Martin 1970, Petersen 1970, Drut et al. 1994a, 1994b). At 12 weeks of age,
insects only make up 5% of the diet (Peterson 1970). Sage Grouse generally obtain
27
enough moisture from the food they eat, but in dry years they have been observed
drinking from standing water (Patterson 1952; C. L. Aldridge, pers. observ.).
Survival
Annual adult survival has been estimated at 30 to 75% with males having lower
survival rates than females (June 1963, Johnsgard 1973, Braun and Beck 1985, Schroeder
et al. 1999). Adult males often have lower survival rates than do yearlings (Braun and
Beck 1985, Schroeder et al. 1999), likely due to the costs incurred during courtship
(Braun and Beck 1985). Dalke et al. (1963) observed a female that had been banded
seven years earlier. Juvenile mortality may be high, with large numbers succumbing to
disease and predation (Patterson 1952). Major predators include hawks, falcons (Falco
spp. and Accipiter spp.), eagles, coyotes (Canis latrans), and red fox (Vulpes vulpes)
(Schroeder et al. 1999). Ground squirrels (Spermophilus spp.), striped skunks (Mephitis
mephitis), Black-billed Magpies (Pica pica), Common Ravens (Corvus corax), American
Crows (Corvus brachyrhynchos) and coyotes prey upon eggs and chicks (Patterson 1952,
Schroeder et al. 1999). Both the red fox and raccoon (Procyon lotor) have increased in
numbers on the Canadian prairies and may also be important nest predators (Aldridge
1998; W. C. Harris, pers. commun.). Crawford and Lutz (1985) found that survival rates
of adults in Oregon were similar from 1941 to 1983, even though the population declined.
LIMITING FACTORS
Although many different factors may have contributed to a reduction in Sage
Grouse numbers throughout the species’ range, most deal with loss of suitable habitat
28
(Patterson 1952, Eng and Schladweiler 1972, Braun 1995, Braun 1998, Schroeder et al.
1999) and the degradation and fragmentation of remaining habitat (Schroeder 1997,
Braun 1998, Schroeder et al. 1999). These alterations due to human encroachment and
development as well as changes in climate and predator communities all may affect
survival and productivity. A number of more localized disturbances, such as industrial
development, have also contributed to the loss of suitable habitat. These potential
limiting factors are addressed below.
Agricultural Practices
The demand for productive agricultural land in the 20th century resulted in
massive sagebrush eradication programs. This decreased the range of sagebrush and,
thus, potential Sage Grouse habitat by an estimated two and a half million ha from 1952
to 1977 (Braun et al. 1977). Cultivation of sagebrush-grasslands has directly resulted in
the desertion of at least one lek in Alberta (Dube 1993), and possibly a second (C. L.
Aldridge, pers. observ.). Ploughing in Montana reduced Sage Grouse habitat by 16%,
including 30% of the wintering range, and the population declined by 73% (Swenson et
al. 1987). In addition, birds moving to forage in crop fields can be killed or injured by
machines and other farm equipment (Patterson 1952; C. L. Aldridge, pers. observ.).
Insecticides and herbicides are potentially lethal to Sage Grouse (Blus et al. 1989),
although their use on Canadian rangelands is limited.
Overgrazing has long been suggested as one of the main reasons for declining
Sage Grouse numbers (Dalke et al. 1963, Johnsgard 1973, 1983). The declines indicated
by lek counts in Alberta since 1968 correspond with an increase in number of livestock
29
grazing in the southeastern part of the Province (Windberg 1975). The removal of cover
by cattle can impact Sage Grouse populations either by reducing habitat suitability, or by
increasing the exposure of birds to predators or extreme weather. Grazing may simply
decrease the carrying capacity of Sage Grouse habitat (Windberg 1976), especially in
years with below average annual precipitation.
Human Disturbance
The unique spring mating rituals of Sage Grouse attract naturalists, researchers,
and interested members of the public each year. Nature photographers set up blinds at
leks each spring, in an attempt to photograph male Sage Grouse in full breeding display.
However, if birds are disturbed at leks, individuals will not return until the next day (C.
L. Aldridge, pers. observ.). Continual disturbance at leks could result in abandonment of
that site, and may ultimately have detrimental effects on breeding success and survival of
the population.
Predation
The predator community on the Canadian prairies has undergone drastic changes
in the last 150 years. With the loss of the Plains Grizzly Bear (Ursus horribilis) and the
Plains Wolf (Canis lupis), the coyote is now the top predator. The swift fox (Vulpes
velox) was extirpated from Canada, although a small population now exists after an
extensive reintroduction effort. Raccoons, striped skunks, and red fox, have all increased
on the prairies, especially in last half century (see Survival section). Richardson’s ground
squirrels are potential nest predators of Sage Grouse. While Common Ravens do not
30
coexist with Sage Grouse in Canada, American Crows and Black-billed magpies are
common on the prairies and may be nest predators. These changes in the predator
community, combined with alterations in habitat structure and/or species composition,
may have resulted in increased predation pressure.
Oil and Gas Exploration
Oil and gas exploration and extraction within the Canadian range of Sage Grouse
is a key component of the economies of both Alberta and Saskatchewan. The removal of
vegetation for well sites, access roads, pipelines, and associated facilities can reduce and
fragment suitable habitat. Human and mechanical activities at well sites may disrupt
breeding and nesting activities. Even if sites are reclaimed, birds often fail to return to
use these areas as leks, as has been the case for at least one site in Alberta (C. L.
Aldridge, pers. observ.). A total of six traditional strutting grounds in Alberta have been
disturbed by oil and gas activities in the past, four of which are no longer active (Dube
1993; Aldridge 1998; C. L. Aldridge, pers. observ.).
The construction of power lines to well sites and pump jacks and compressor
stations provide perching sites for raptors, likely increasing the risk of predation. Power
lines and roads tend to fragment habitat, providing corridors for predators such as
coyotes, and making habitat less suitable to Sage Grouse. Sage Grouse have also been
killed flying into power lines (Borell 1939; Patterson 1952; C. L. Aldridge, pers. observ.).
31
Roadways and Traffic
Heavily used roads and highways result in Sage Grouse mortalities (Patterson
1952). Sage Grouse travel on the ground to and from leks and foraging sites, and many
individuals are killed by vehicles (Patterson 1952). Some leks are located on frequently
traveled roads (Patterson 1952), which can have obvious detrimental effects. In addition,
roadways may render leks more visible to humans, which could lead to abandonment if
breeding activities are continually disturbed.
Climate
Although Sage Grouse are fairly robust birds, harsh climatic conditions at the
northern edge of the species’ range may affect populations. Short summers and
particularly harsh winters likely reduce the ability of individuals to find enough food in
winter months and decrease lipid reserves necessary for reproduction (Back et al. 1987,
Hupp and Braun 1989a) and possibly lower overwinter survival (Back et al. 1987).
There is a positive relationship between spring precipitation and Sage Grouse
productivity (Gill 1966, Chapter 4). Years with below average spring moisture result in
less vegetation growth, potentially reducing Sage Grouse nest success and limiting the
availability of lush vegetation that is an important summer dietary component for Sage
Grouse, especially chicks. During the 1980s, spring precipitation was considerably lower
than the long-term average (Fig. 4.2; Onefour Research Station, Environment Canada).
This likely contributed to decreased productivity and survival (Chapters 3, 4).
The effects of other limiting factors may be compounded during drought
conditions. For example, consistent cattle stocking rates during the drought of the 1980s
32
may have resulted in a substantial loss of vegetative cover, perhaps lowering nest
success, increasing predation, and possibly lowering overwinter survival (K. J. Lungle,
pers. commun.). Impacts may have been particularly severe in more moist habitats,
which supply important herbaceous growth during nesting and brood rearing. The
attraction of cattle to these areas was probably increased during drought conditions,
which may have decreased brood survival.
SPECIAL SIGNIFICANCE OF THE SPECIES
When Sage Grouse were hunted in Canada, they were hunted as a trophy species,
rather than for food. Sage Grouse have no current commercial value other their value to
the eco-tourism industry, and as an indicator of the health of the prairie ecosystem. With
the increasing number of species in peril on the Canadian prairies, interest and concern
about Sage Grouse continues to increase. The concern stems from the need to understand
the reasons for the continued population decline, and to understand threats that are related
to the real and imminent extirpation of Sage Grouse and other species of concern native
to the prairie ecosystem.
In part, public interest in Sage Grouse stems from the mating rituals that birds
perform each spring at strutting grounds, which are a unique sight on the prairies. With
the decline in population numbers, there is an ever-increasing interest by naturalists,
biologists, and the general public to observe these displays. Sage Grouse are particularly
sensitive to disturbances, and thus, may be a good indicator of the general health of the
prairie ecosystem. The Canadian population may offer scientists an opportunity to
understand strategies that have enabled species to exist at the fringe of their range.
33
EVALUATION
Connelly and Braun (1997) reported that range-wide decreases from prior to 1985
to after 1995 averaged 33% (range 17 to 47%). Braun (1998) suggested that overall
breeding populations have declined by 45 to 80% since the early 1950s. The decline may
have been the most drastic in Canada over the last 30 years; lek counts indicate a decline
of between 66 and 92% in Alberta since 1968. The species now occurs in less than 10%
of its historic prairie range within Canada. As a result of declining population numbers,
the species’ limited distribution, and specific habitat requirements, COSEWIC listed Sage
Grouse as an Endangered species in 1998 (Hyslop 1998).
Oil and gas activities continue to threaten remaining habitats, and pose many
different hazards. Activities in important breeding, nesting, and brood rearing areas, or
the alteration of key wintering habitat may have profound negative effects on the
population. Combined with recent changes in the predator community, and the loss of
suitable nesting and escape cover related to cattle grazing and recent drought on the
prairies, changes in land management practices may be needed for the continued
existence of these birds in the Canada.
34
CHAPTER 2.
NESTING AND REPRODUCTIVE ACTIVITIES
35
INTRODUCTION
Sage Grouse (Centrocercus urophasianus) across their range have experienced
declines ranging from 45 to 80% since the 1950s (Braun 1998) with the majority of the
decrease in population size occurring since 1980. The historical declines are attributed to
human induced reduction in sagebrush (Artemisia spp.) habitats (Patterson 1952, Eng and
Schladweiler 1972, Braun 1995, Braun 1998, Schroeder et al. 1999). However, many
other factors have altered and fragmented current habitat and may also have contributed
to recent population declines. These include the construction of roads, highways, fences
and power lines, domestic livestock grazing, and a variety of natural changes [reviewed
by Braun (1998) and Schroeder et al. (1999)]. Although these changes affect all aspects
of Sage Grouse life history, variation in productivity has been proposed as the most
important factor regulating all grouse populations (Bergerud 1988). In some populations,
survival has remained relatively constant, while productivity has declined; as for Sage
Grouse in Oregon (Crawford and Lutz 1985). Thus, the majority of research on grouse
has focused on the relationship between measures of productivity and population
dynamics (Bergerud 1988, Schroeder 1997, Schroeder et al. 1999). More recently, there
have been attempts to relate productivity to measures of habitat quality (Schroeder 1997;
Sveum et al. 1998a, 1998b; Schroeder et al. 1999).
Productivity is often considered in terms of reproductive effort and reproductive
success. For the purpose of this study, I define reproductive effort to include nesting
effort or attempt(s) and clutch size, whereas reproductive success includes breeding
success, nest success, and fledging success (see definition below). However, productivity
measures should go one step further and include data on the number of young produced
36
during the breeding season that survive to age of independence and reproductive maturity
(recruitment; Gill 1990). Thus, I consider productivity to include reproductive effort,
reproductive success, and recruitment.
Most research on Sage Grouse has shown that population declines are related to
reduced nest success (Schroeder 1997, Braun 1998, Schroeder et al. 1999). Stable
populations tend to have high nesting success (35–86%) (Dalke et al. 1963; Schroeder et
al. 1999; J. W. Connelly, pers. commun.). However, the same factors that affect nesting
success, may also affect chick survival and juvenile overwinter survival (recruitment).
Little research has been conducted to address issues such as chick survival, primarily due
to the difficulties in estimating brood size, and technological limitations making it
difficult to affix radio transmitters to juvenile birds.
Sage Grouse are associated with big sagebrush (A. tridentata spp.) throughout
most of their range; however, the dominant species in Canada is silver sagebrush (A.
cana) (Aldridge 1998). Silver sagebrush is not as tall or as dense as big sagebrush and,
thus, does not provide the same extent of cover for nesting or to escape from predators.
Nesting success is higher in areas containing big sagebrush (Wallestad and Pyrah 1974,
Connelly et al. 1991, Gregg et al. 1994, Young et al. 1994, DeLong et al. 1995, Schroeder
et al. 1999). As a result, the apparently lower quality habitat at the northern fringe of the
species range in Canada may limit the population, which might be expected to fluctuate
to a greater extent than core populations.
Sage Grouse in Canada have declined to less than 20% of the numbers recorded
in the late 1960s in Alberta and the mid 1980s in both Alberta and Saskatchewan
(Aldridge 1998, Chapter 1). It has been suggested the decline is related to poor survival
37
and/or poor productivity; specifically low nesting success. There has been no recent
research on Sage Grouse in Canada to address either survival or productivity. I estimated
the 1999 Canadian spring population to be 813-1204 individuals (Chapter 1). An average
of 15 males were counted on 18 active leks spread over approximately 10,000 km2 in
Canada (Chapter 1; Aldridge 1998; W. C. Harris, Pers. Commun.). This represents a
density of less that one bird/km2 in potential habitat in Canada. Density estimates for
Sage Grouse in other areas are as high as 15 birds/ km2 (Patterson 1952, Wallestad 1975,
Schroeder et al. 1999). The purpose of this Chapter is to compare the timing of
reproductive activities and measures of productivity for a declining Sage Grouse
population in apparently sub-optimal habitat at the periphery of the species range, with
measures of productivity for other declining (Colorado, Idaho, and Montana) and
drastically reduced (Washington) populations. Assuming adult survival is comparable to
other populations, I hypothesize the decline observed in the Canadian Sage Grouse
population is related to reduced reproductive effort, reduced reproductive success, or a
combination of both.
METHODS AND MATERIALS
I monitored the reproductive activities of Sage Grouse within a 4,000 km2 area of
southeastern Alberta (49° 35’ N, 110° 50’ W). Silver sagebrush is the dominant shrub
and pasture sage (A. frigida) the dominant forb. Grasses include speargrass (Stipa
comata), june grass (Koeleria macrantha), blue grama (Bouteloua gracilis), and western
wheatgrass (Agropyron smithii) (Aldridge 1998).
38
I captured females at seven different mating grounds (leks) using walk-in traps
(Schroeder and Braun 1991) and with a long handled hoop net and handheld spotlights
(Giesen et al. 1982) from March through May 1998 and 1999. Several other females
were captured by nightlighting flocks of broodless females in summer. Sex and age
[(yearlings < 2 years old) and (adults ≥ 2 years old)] of all captured individuals were
assigned based on the shape and length of the outermost primaries of each bird (Eng
1955, Crunden 1963, Braun 2000). Captured females were fitted with a 14 g necklace
style radio transmitter (RI-2B transmitters; Holohil System Ltd.; Carp, Ontario).
Females were located every second day using a 3-element Yagi antenna and
portable receiver (TR2 scanning receiver, Telonics Inc., Mesa, Arizona; Merlin 12
receiver, Custom Electronics of Urbana, Inc., Urbana, Illinois). Standard telemetry
techniques were used to determine location of females. Signals were triangulated until
birds could be observed from approximately 30 m away. These locations were recorded
in Universal Transverse Mercator Coordinates using a hand held 12 Channel Global
Positioning System (Garmin 12 XL and GPS II Plus units; Garmin International Inc.,
Olathe, KS). When signals disappeared, I searched the entire study area from a fixedwinged aircraft.
Measures of reproductive success were calculated in a manner similar to
Schroeder (1997). Clutch size was estimated by counting the number of egg shells
following either successful hatch or after destruction of the nest. In all cases, egg shells
were counted within 3 days of hatch or nest destruction. I estimated the date of nest
initiation as the midpoint between the last observation in which the female did not display
localized movements and the first direct observation of the hen on a nest. Nest success
39
was defined as the percentage of nests that hatched ≥ 1 egg. Date of nest success or nest
failure was estimated as the midpoint between the last observation of the female on the
nest and the first observation in which she was off the nest. Breeding success was
defined as the percent of females that hatched ≥ 1 egg during a single breeding season
(first or renest). Fledging success was the percent of females that raised at least one
chick to independence (assumed ≥ 50 days of age for comparison with other studies;
Schroeder 1997). This parameter can be calculated relative to either the number of
females that made nesting attempts (Schroeder 1997), or compared to only those females
that successfully hatch at least one egg (successful breeders). The latter differentiates
between nest success and survival of the brood. I calculated fledging success relative to
successful breeders only, because females that are unsuccessful nesters inherently cannot
successfully fledge young. However, I also calculated fledging success for all females
that attempted to nest, to allow for comparison with other studies. Lastly, I estimated
chick survival as the percent of young that lived ≥ 50 days.
Due to a limited sample size of nesting females in 1998 and nesting yearlings
throughout the study, I was limited in my analyses of year and age effects. I tested for
age related differences in reproduction and timing of reproductive activities, and also for
differences in clutch size, nest success, and timing of reproductive events, as they related
to first and renesting attempts. Analyses were conducted using one-way Analysis of
Variance for clutch size and timing of reproductive events, and a Chi-square test was
used to analyse nesting success, fledging success, and breeding success. I conducted all
analyses useing an " value of 0.05. My productivity measures were then compared with
estimates from other populations.
40
RESULTS
I captured thirty-seven female Sage Grouse on leks and fitted them with radio
transmitters; 2 adults and 3 yearlings in 1998; 22 adults and 10 yearlings in 1999. Three
additional adult females were captured in summer 1998 and 1 in summer 1999. Two
females captured in summer of 1998 were eaten by a predator before the 1999-breeding
season and the female captured in summer 1999 had a brood, but was depredate two
weeks after capture.
I collected reproductive data for 20 different individual females; 3 females in
1998; 19 females in 1999, 2 of which I collected data in both years. Of a total of 41
radio-marked females, 5 radios apparently malfunctioned and 4 females died prior to the
breeding season. I could not relocate 12 of the remaining 32 females during the breeding
season; 4 were found dead with damaged transmitters in late spring or early summer and
were likely died before nesting; 1 was captured with chicks after nesting; 1 was
recaptured on 23 July 1999 with a brood patch but no brood. The fate of 6 females is
unknown.
The mean date of capture for 36 females caught during the breeding season was 8
April ± 1.9 days (SE), while the mean date of maximum attendance of females (breeding)
at 7 leks in 1999 was 5 April ± 0.91 days. This suggests the peak in breeding occurred
during the first week of April (1998 data were not used to calculate maximum attendance
dates due to limited counts at leks prior to 11 April; one lek in 1999 was not used in the
analyses due to limited observations early in the breeding season).
41
Initiation of Incubation
The mean initiation date of incubation for 25 nests was 10 May (range 22 April to
18 June). Initiation dates were based on restricted movements of females, or the
observation of a female under the same shrub on consecutive relocations. Age (adult vs.
yearling; F1,23 = 0.004, P = 0.95) did not explain a significant amount of variation in the
dates of incubation initiation. Age, however, did explain a significant amount of the
variation (F1,18 = 7.34, P = 0.01) for first nesting attempts only. Average date of first nest
incubation for adult females (1 May, range 23 April to 10 May) was earlier than that for
yearlings (10 May, range 5 to 14 May). Incubation of first nests began approximately 35
days earlier than renesting attempts (F1,23 = 113.45, P < 0.001).
Incubation lengths ranged from 23 to 29 days (0 = 27 ± 0.6 days (SE), n = 10)
and was similar for both first nests and renesting attempts (F1,8 = 0.97, P = 0.35). Hatch
date for successful nests (0 = 5 June ± 4.59 days, n = 12) was significantly earlier (33
days) for first nests (0 = 28 May ± 1.58 days, n = 9) than for renesting attempts (0 = 30
June ± 5.36 days, n = 3; F1,10 = 68.97, P < 0.001).
Clutch Size
Clutch size for 28 nests ranged from 4 to 11 eggs. Nest success was independent
of clutch size (0 = 7.8 ± 0.36, F1,26 = 0.46, P = 0.50), therefore I pooled successful (0 =
8.0 ± 0.53, n=12) and unsuccessful nests (0 = 7.6 ± 0.48, n = 16) for analyses. First nests
(0 = 8.2 ± 0.24 eggs, n = 23) had larger clutches than renesting attempts (0 = 5.6 ± 0.70
eggs; n = 5, Fig. 2.1, F1,26 = 12.38, P < 0.002). Egg viability (% of all eggs laid in
successful nests that hatched at least one egg) was 92% (96 of 104 eggs).
42
10
Clutch Size
8
6
4
2
(23)
(5)
First Nest
Renest
0
Figure 2.1. Clutch size for first nests and renesting attempts by Sage Grouse in
southeastern Alberta in 1998-99. Values are shown as means ± one standard
error. Number of nests is indicated in brackets.
Nesting Attempts
I based my estimate of annual nesting effort on data from 22 females. In all cases,
females displayed localized movements within an area and a nest was eventually located
after a successful hatch or a predation event. Nesting effort was 100%, with all females
attempting to nest.
Nest Success
Overall nest success (percent of all nests that hatched ≥ 1 egg) was estimated at
46.2% for 26 nests (1/3 nests in 1998, 11/ 23 nests in 1999). This is actual nest success.
I did not use a Mayfield estimator as all females that were tracked attempted to nest
(Mayfield 1975). Nest success was independent of nest order (first nest attempt or renest;
X2 1 = 0.26, P = 0.61) and female age (X2 1 = 0.46, P = 0.50) although adults (11/22, 50%)
43
were more successful than yearlings (1/4, 25%). The majority of nest failures occurred in
mid-incubation (Fig. 2.2). As a result, I grouped nest failure into four
Nest Survival (%)
100
80
60
40
20
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27
Days After Nest Initiation
Figure 2.2. Kaplan-Meier product limit estimate for nest survival in relation to date after
initiation, for Sage Grouse nests in southern Alberta in 1998 and 1999. The 95%
confidence limits are shown by the dashed lines.
separate 7-day stages of incubation (Fig. 2.3). The majority of nest failures occurred
between day 8 and 21 of incubation (86%, 12/14 failures). If number of nests entering
each time period is considered, the percentage of nests that were destroyed within each
stage of incubation was also highest during the second and third weeks of incubation
(Fig. 2.3).
44
Nests Destroyed (%)
40
35
30
25
20
15
10
5
0
1-7
8-14
15-21
22-28
Days After Initiation of Incubation
Figure 2.3. Timing of nest failure for 14 unsuccessful Sage Grouse nests in southeastern
Alberta in 1998-99. Percentage of nest failures within each of the four categories
is calculated based on the number of nests entering that period (including
successful nests).
Renesting Likelihood
Only 4 of 13 (30.8%) females that were unsuccessful in their first nest attempts in
1999 renested (4 of 10 adults and 0 of 3 yearlings). Despite the fact that no yearlings
attempted to renest, there was no statistically significant age effect on renesting
likelihood (X2 1 = 1.20, P = 0.27). In 1998, one adult female was captured at a lek late in
the breeding season (22 May) with a well developed brood patch. I assumed she had
failed in her first nesting attempt. She did initiate a second nest that year, but was
unsuccessful.
45
Breeding Success
Breeding success was estimated to be 54.5% for 22 females monitored throughout
a single breeding season. Breeding success was independent of age (X2 1 = 0.78, P =
0.38; adults = 61%, n = 18; yearlings = 25%, n = 4) although adults appeared to be more
successful at hatching at least one egg than yearlings.
Fledging Success
I estimated fledging success to be 22.7% for females that attempted to nest. This
was independent of age (X2 1 = 0.91, P = 0.34; adults = 27.8%, n = 18; yearlings = 0%, n
= 4). Fledging success calculated for successful breeders was 41.7% (n = 12 females).
This was independent of age (X2 1 = 0.33, P = 0.56; adults = 41.7%, n = 11; yearlings =
0%, n = 1). Using both methods, fledging success was statistically independent of age,
despite the fact that no yearlings successfully fledged chicks. However, these differences
are based on only four yearlings.
Chick Survival
Due to the difficulty in locating all chicks when a brooding female was flushed, I
estimated chick survival to 50 days of age as a range (minimum and maximum). Chick
survival ranged from 13.6 to 22.7% for 88 chicks, with no yearling females successfully
rearing chicks to fledge (≥ 50 days of age).
46
DISCUSSION
Initiation of incubation ranges from late March to mid-May across the species’
range (Dalke et al. 1963, Schroeder 1997). Mean nest initiation date was 10 May in my
study; 3 May for first nesting attempts. Thus, nesting activities occur later in Canada,
even when compared to Sage Grouse with only a slightly more southern latitude in
Washington; mean date of incubation initiation in Washington was 22 April (Schroeder
1997). Sage Grouse are at the northern extent of their range in Canada and longer
winters may reduce the length of the breeding season and result in later nesting events
compared to the rest of their range. Combined with the shorter growing season and the
different species of sagebrush, habitat and conditions might always be less suitable in
Canada compared to the rest of the species’ range.
It is possible that trapping Sage Grouse affects nesting efforts and timing of
nesting activities. However, only three individuals trapped in 1998 were relocated in
1999, which did not allow me to test for such effects. Schroeder (1997) found that newly
captured female Sage Grouse in Washington nested, on average, one day later that those
captured the previous year.
Mean date of hatch in my study was 28 May for first nesting attempts, and 30
June for renests. Kerwin (1971) estimated the mean hatch date for Sage Grouse in
Saskatchewan to be in the first or second week of June, which is comparable with my
data. The mean date of breeding activities (peak attendance of females at leks) for Sage
Grouse ranges from late March to late April (Schroeder et al. 1999, see Chapter 1,
Population Trends), and is typically later at more northern latitudes and higher elevations
(Petersen 1980). Kerwin (1971) for Saskatchewan and Clewes (1968) for Alberta both
47
found the peak in breeding occurred at the end of April. The peak in breeding activities
in my study occurred in the first week of April, earlier than estimates for Sage Grouse in
Canada. These previous studies may have missed the peak in breeding activities or
females may have attempted to renest more often, increasing female attendance at leks
later in the breeding season.
Incubation length typically ranges from 25 to 29 days (Patterson 1952, Schroeder
1997, Schroeder et al. 1999) but some estimates are as low as 20-24 days (Schroeder et
al. 1999). In my study, mean incubation length was 27 days (range 23-29). Incubation
was estimated to be 23 days for 2 females; however, both individuals could not be located
for several days prior to being observed on the nest, and thus, the incubation time for both
is likely underestimated.
Mean clutch size in my study was 7.8, which is high compared to most other
studies in North America (typically between 6.6 and 8.2) (Patterson 1952, Wallestad and
Pyrah 1974, Petersen 1980, Connelly et al. 1993, Gregg et al. 1994, Schroeder et al.
1999). This was also likely an underestimate, as one female that only had 4 eggs in her
nest when it was destroyed, had 6 eggs in her nest 10 days earlier when accidentally
flushed (I assumed a clutch of 6 eggs for this female). Similar inferences about clutch
sizes being underestimated have also been reported by Schroeder (1997) and it is possible
that Richardson’s ground squirrels (Spermophilus richardsonii), American Crows
(Corvus brachyrhynchos), or Black-billed Magpies (Pica pica) removed eggs during
incubation, or removed shells after nest failure or hatch. Schroeder (1997) found that
Sage Grouse in Washington had unusually high clutch sizes (0 = 9.1 ± 1.30, n = 55).
Clutch size may be correlated with nutrition (Lack 1968). Schroeder suggested that
48
nutrition is likely a direct reflection of habitat quality, but a relationship between clutch
size and nutrition has yet to be illustrated for any grouse species (Schroeder 1997).
Considering that egg production and laying begin in northern grouse populations (where
clutch size tends to be larger) prior to the time of new plant growth, spring growth and
nutrition should have little effect on clutch size (Bergerud 1988), especially in highly
fragmented habitats, as was the case in Schroeder’s (1997) study area.
It has been suggested that clutch size for grouse is negatively correlated with
annual survival of breeding-age individuals (Bergerud 1988). Bergerud further suggested
that grouse with annual survival rates of ≥ 50% had low clutch sizes (5-8 eggs), and
grouse with survival rates ≤ 45% had large clutches (9-13 eggs). He based his hypothesis
on a Sage Grouse population with a 40% survival rate for breeding age individuals and a
clutch size of about 8. Schroeder (1997) found survival to be high (55 to 75%) and clutch
sizes large (0 = 9.1) and concluded that the Washington Sage Grouse population did not
fit Bergerud’s hypothesis. Survival of females in my study was below 56.5% if potential
fall and winter mortality is considered (Chapter 4, Table 4.1). Thus, with a relatively
high clutch size of 7.8 eggs per nest (8.0 for successful nests) and higher survival rates
than in Bergerud’s (1988) synthesis (40%), my data support Schroeder’s contention that
Sage Grouse clutch size is not negatively correlated with survival. However, my data for
survival by breeding age males (31%) and females (≤ 56.5%) are both lower than
typically reported for Sage Grouse (males 38-60%, females 55-75%) (Schroeder et al.
1999), yet clutch size was at the high end of the range for Sage Grouse (6.6-9.1)
(Schroeder et al. 1999). Bergerud (1988) suggested that clutch size should also decrease
with increasing risk of predation. Schroeder (1997) reported high levels of nest predation
49
in his study (nest success was low at 36.7%) and, thus, clutch size should have been
lower rather than increased, due to the high risk of predation.
Clutch size in some populations of Sage Grouse is age-specific, with adults
having larger clutches (Wallestad and Pyrah 1974, Petersen 1980), although Schroeder
(1997) found no differences. My sample sizes do not permit a test of whether adults
produced larger clutches than yearlings. Capture rates for Sage Grouse indicate that
yearlings are under represented in the population (3 to 1 adult to yearling ratio; Chapters
1, 4). Assuming that adults produce larger clutches than yearlings, the large proportion
of adults in this population may have influenced mean clutch size in my study.
Previous research using telemetry has shown that in any year, 20 to 32% of
females do not nest (Wallestad and Pyrah 1974, Connelly et al. 1993, Gregg et al. 1994,
Schroeder et al. 1999). However, my results are consistent with Schroeder’s (1997)
findings for Washington, which indicate that all females make nesting attempts. Based
on follicular development, 98% of 338 females in Idaho (Dalke et al. 1963) and 91% of
395 females in Colorado (Braun 1979) ovulated, suggesting that nearly all females mated
and attempted to nest. It is likely that previous telemetry studies have underestimated
nesting attempts. Reproductive activities may also be influenced by intrinsic factors such
as density dependence and local survival rates (Bergerud 1988). High densities could
conceivably result in competition by females for nest sites, to the point of precluding
some females from nesting. In low density populations (e.g., Washington and Canada),
competition for nests sites may not exist and, thus, all females attempt to nest.
Two studies have shown that Sage Grouse select nest sites independently of lek
location (Bradbury et al. 1989, Wakkinen et al. 1992). However, while nest locations
50
may be random with respect to lek locations (Wakkinen et al. 1992), the average distance
that females nest from the lek where they were captured ranges from 4.0 km in Colorado
(Petersen 1980) to 7.8 km in Washington (Schroeder et al. 1999), with the majority of
nests occurring within 3.2 km of a lek (80% within 3.2 km, Martin 1970; 92% within 3.0
km, Wakkinen et al. 1992; and 68% within 2.5 km, Wallestad and Pyrah 1974). In areas
where habitat is more fragmented and of poor quality, nest success and survival would be
expected to decline. However, habitat quality could indirectly affect reproductive efforts,
including both nesting and renesting likelihood. Poor quality habitat could force females
to search over larger areas for suitable nesting sites. It has been suggested that nest-lek
distances may be inversely correlated with habitat quality (Autenrieth 1981). The largest
known mean nest-lek distance of 7.8 km was found in a highly fragmented habitat in
north-central Washington (Schroeder et al. 1999).
Average nest to lek distance in my study was 4.7 km (± 0.66 km; range 0.4215.45 km) with only 40.7% of 27 nests occurring within 3.2 km of the lek. This suggests
that suitable nesting habitat within the Canadian range may be limiting, and females are
moving considerable distances to locate suitable habitat. The costs associated with
extended search efforts and increased movements to locate suitable nest sites could result
in reduced reproductive effort and/or success. This did not appear to be the case in
Washington as all females attempted to nest; however, it may have affected nesting
success, which was low (36.7%) (Schroeder 1997). Although Sage Grouse in my study
moved greater distances to locate suitable nest sites, all females attempted to nest, had
relatively large clutches, and were fairly successful nesters, when compared to other
populations.
51
Based on the population estimates from my study, I conclude that reproductive
effort is not the cause of the decline of the Canadian Sage Grouse population. All
females attempted to nest and clutch size was within the expected range.
In a recent review, Schroeder et al. (1999) showed that Sage Grouse nest success
ranges from 15-86% throughout North America. In nine of fourteen studies for which
there are productivity data, nest success was between 30 and 60%. Nest success in my
study (46.2%) is within this range. However, spring precipitation (April - June) was
above average in both years of my study (1998 = 152.5 mm, 1999 = 159.5 mm, 19651999 0 = 133.5 mm; Onefour Research Station, Environment Canada). Gill (1966) found
a weak positive correlation between nest success and spring precipitation, suggesting the
increase in spring moisture likely resulted in above average nest success in my study.
The probability of renesting by radio-marked Sage Grouse in my study was
30.8%. Renesting varies from 5 to 87% for other populations (Schroeder et al. 1999).
Schroeder (1997) suggested that estimates of renesting and nest success would be
negatively correlated with a researcher’s ability to find all nests, as successful nests are
easier to locate. If nests that have been destroyed prior to incubation are not located, nest
success and renesting attempts will be underestimated. It is possible that I missed some
first nesting attempts, underestimating nesting effort. However, most females that were
tracked after their capture, displayed localized movements and, in all cases, a nest was
subsequently found.
The mean capture date of females on leks was 8 April. Most females returned to
the lek for approximately 3 days after capture (C. L. Aldridge, pers. observ.) and, thus, 11
April is a reasonable date to assume that females began searching for suitable nest sites.
52
Patterson (1952) suggested that egg laying begins a few days after females move from
leks to nesting areas. Thus, I estimate that the mean date of egg laying for initial nests
based on lek attendance would be 16 April. With a mean clutch size of 8.2 for first nests,
and 1.3 days between the laying of successive eggs (Patterson 1952), egg laying should
last approximately 11 days. Combined with mean incubation time of 27 days, egg laying
and incubation would last approximately 38 days for first nests. Thus, based on peak
attendance of females at leks, mean hatch date for first nests should have occurred on
approximately 24 May. Mean hatch date for first nesting attempts was 28 May, only four
days after that predicted by female movements. This strongly supports my contention
that I did not miss any first nesting attempts. However, the estimated 38 days from
breeding to hatching of clutches is probably minimal.
Bergerud and Gratson (1988) hypothesized that renesting in grouse should be
negatively correlated with female life expectancy; similar to Bergerud’s (1988)
hypothesis that clutch size was negatively correlated with life expectancy. It is
conceivable that nesting likelihood is also negatively correlated with survival. Sage
Grouse are relatively long lived compared to other grouse (Patterson 1952, Bergerud
1988, Bergerud and Gratson 1998) and, if the hypothesis is true, should exhibit low
renesting rates. However, survival of females in my study was less than 56.5%, which is
low compared to female survival estimates in other populations (range 55-75%;
Schroeder et al. 1999). Therefore, given the low survival probability, renesting rates of
Sage Grouse in Canada should be high, yet I found only 30.8% of females attempted to
renest. These data do not support the hypothesis that renesting is negatively correlated
with life expectancies of breeding-age females (Bergerud and Gratson 1988). Schroeder
53
(1997) found that Sage Grouse in Washington also do not fit the renesting hypothesis,
with high renesting rates (87%) and high annual female survival (55-75%).
Survival estimates in this population may be underestimated due to the biases
associated with radiotelemetry studies. However, survivorship for other populations was
also ascertained using radiotelemetry and thus, my estimates should be comparable.
I expected that nests have an overall greater likelihood of hatching the longer they
survived, because vegetation structure should increase, better concealing nests from
predators. This effect should be enhanced over the course of this study due to the moist
springs of 1998 and 1999. This was the case, with nest failure greatest during the second
week of incubation and declining over the third and fourth weeks of incubation (Figs. 2.2,
2.3).
Breeding success has been reported to range from 15-70% (Schroeder et al.
1999). In my study, breeding success (54.5%) was higher than reported in 6 of 8
previous studies (see Schroeder et al. 1999). This implies that breeding success is not
limiting the Canadian population. However, similar to nest success, breeding success
may have been elevated over the course of my study due to higher than average spring
precipitation.
Overall, nest success and breeding success for Sage Grouse in Alberta were
comparable to other populations. Fledging success estimated from those females that
were successful breeders was 41.7%, indicating that less than half of all broods had at
least one chick survive to the fall. However, when compared to Schroeder’s (1997)
estimate of fledging success calculated from all nesting attempts (49.5%), fledging
success in my study was lower (27.7%), despite high nesting success. Female Sage
54
Grouse tend to flock together with other females when they loose their broods (C. L.
Aldridge, pers observ., C. E. Braun, Pers. Commun.). Thus, I am confident that brood
survival was not underestimated. This suggests that low overall brood survival could be
causing decreased reproductive success for Sage Grouse in Canada.
Brood counts are often used to estimate chick survival for grouse. However, it is
often difficult to locate all chicks associated with a brood, and a direct relationship
between brood flush counts and actual brood size has yet to be shown. Brood size for
Sage Grouse declines during the summer by as much as 68.4% (Schroeder et al. 1999),
which may reflect the characteristic low survival rates for juveniles. However, Schroeder
(1997) estimated chick survival in Washington at 33.4% and June (1963) reported that
38% of Sage Grouse chicks in Wyoming survived to the autumn. Both populations
showed slight decreases over the course when the studies were performed (June 1963; M.
A. Schroeder, pers. commun.). This suggests that chick survival of at least 35% may be
required to sustain a Sage Grouse population, assuming reasonable levels of reproductive
effort and reproductive success. Even though reproductive effort, and, for the most part,
reproductive success (fledging success was low) for Sage Grouse in Canada is
comparable to other studies, chick survival to 50 days of age (14-23%) was less than half
of the estimated 35% survival required for stable populations.
Spring precipitation was above average over the course of my study. While
spring precipitation has been linked to increased nest success (Gill 1966), Peterson
(1970) also found that wet years resulted in greater forb production and increased brood
success in Montana. My estimates of chick survival could also be elevated due to
increased spring precipitation.
55
With brood counts indicating that chick survival was extremely low, I expected
low recruitment rates. Several long-term studies that involve capturing Sage Grouse have
shown that between 44% (n = 440; Dalke et al. 1963) and 46% (n = 1371; Braun and
Beck 1985) of captured birds are yearlings. Over the two years of my study, I captured
96 different Sage Grouse and only 25% were yearlings. This suggests that overall
recruitment is low for the Canadian population (Figs. 1.5, 1.6).
From 1996 to 1999 the Canadian Sage Grouse population has remained relatively
stable, at low numbers, despite apparently low chick survival. However, over the course
of my study (1998-99), nest success and breeding success were likely inflated due to
increased spring precipitation (Fig. 4.1) (June 1963, Gill 1966), possibly masking the
effects of low chick survival. The ultimate test of these predictions will be evaluated by
assessing population status in 2000.
I conclude that reproductive effort (clutch size, nesting effort) does not appear to
be related to Sage Grouse population declines in Canada. Reproductive success in this
study, including nesting success, breeding success, and fledging success, was comparable
to other populations, however, these measures of productivity may have been elevated
throughout my study due to above average spring precipitation. Overall, productivity
appears to be limited by low chick survival (14-23% compared to 35% in other
populations). Food availability and quality may influence chick survival (Pyle and
Crawford 1996) as well as clutch size (Lack 1968) and nest success. Habitat quality may
also affect productivity (Sveum et al. 1998a) and may be directly related to food
availability and quality for chicks (Chapter 3). Sage Grouse in Canada exist at some of
the lowest known densities (< 1 bird/km2 ; Chapter 1). The carrying capacity of the
56
population may be limited due to decreased productivity and possibly poor chick
survival, as a result of limited availability of mesic habitats with higher forb availability
(Chapter 3). Over the course of this study, estimates of reproductive effort, nest success,
and breeding success suggest that suitable habitat may be limiting for Sage Grouse in
Canada. These results are based on two years of data, with most data from 1999. In both
of these years, spring precipitation was above the 35-year average, which may have
increased habitat quality and elevated my measures of productivity compared to average
productivity over the last 20 years. Even though this may have been the case, chick
survival was still extremely low, and availability and/or quality of food resources for
chicks, may affect chick survival and/or increase the chance of chick predation (Chapter
3). Overwinter mortality of juvenile Sage Grouse, which was not measured in this study,
may add to the already low estimates of recruitment.
57
CHAPTER 3.
NESTING AND BROOD HABITAT USE
58
INTRODUCTION
The distribution of Sage Grouse throughout North America has been reduced by
at least 50% since the early 1900s. Populations have been extirpated from five of 16
states and one of three provinces (Braun 1998). Population declines averaged 33% from
1985 to 1995 (Connelly and Braun 1997) and declines from 45 to 80% have occurred
since the 1950s (Braun 1998). The most severe declines have occurred at the northern
fringe of the species’ range, where the Alberta population has decreased by 66 to 92%
since 1968 (Chapter 1). The historical range within Alberta and Saskatchewan has been
reduced by approximately 90% (Figure 1.2, Chapter 1).
Long term data on Sage Grouse population trends in Oregon suggest that a 60%
decline in numbers is related to changes in productivity (Crawford and Lutz 1985).
Changes in productivity can be attributed to changes in reproductive effort (nesting effort
and clutch size), reproductive success (nest success, breeding success, fledging success,
chick survival), and/or post fledging mortality (Chapter 2). Many studies of declining
Sage Grouse populations have investigated reproductive effort and measures of
reproductive success. Population declines appear to be linked to nest success and/or
measures of brood survival (Crawford and Lutz 1985, Schroeder et al. 1999). Unsuitable
nesting and brood rearing habitat may contribute to decreases in productivity by reducing
nest success and/or chick survival (Crawford and Lutz 1985, Sveum et al. 1998a).
Sage Grouse tend to place their nests under sagebrush (Artemisia spp., > 90% of
154 nests in Wyoming, Patterson 1952; 91% of 87 nests in Idaho, Klebenow 1969; 79%
of 83 nests in Idaho, Connelly et al. 1991; 71% of 93 nests in Washington, Sveum et al.
1998b). Greater canopy cover of shrubs (primarily sagebrush) is preferred (Patterson
59
1952, Klebenow 1969, Wallestad and Pyrah 1974, Schroeder et al. 1999), particularly
shrubs of medium height (40-80 cm; Gregg et al. 1994, Sveum et al. 1998b). Nests with
greater overall shrub cover are more successful (Wallestad and Pyrah 1974, Connelly et
al. 1991, Sveum et al. 1998b). Tall grass cover is also selected in areas immediately
surrounding nest sites and is positively correlated with nest success (Gregg et al. 1994,
Sveum et al. 1998b). Some nests are placed under shrubs other than sagebrush (Patterson
1952, Klebenow 1969, Connelly et al. 1991, Gregg et al. 1994) and it may be that
females select nest sites based on suitable cover, not by shrub species. Klebenow (1969)
suggested that females might be reacting to the uneven distribution of preferred cover
within the available habitat.
Grouse may or may not select for sagebrush at brood rearing locations (Sveum at
al. 1998a) but forb cover is greater at brood use sites than at random sites, particularly
cover of forbs used as food (Klebenow and Gray 1968, Peterson 1970, Schoenberg 1982,
Drut et al. 1994a, Sveum et al. 1998a). Brood habitat tends to shift from sagebrush
uplands early in the brood rearing period, to more mesic sites later in the summer
(Patterson 1952, Peterson 1970, Autenrieth 1981, Dunn and Braun 1986b). This shift is
usually a result of the desiccation of forbs in sagebrush uplands and an increase in forb
growth at more mesic sites later in the summer (Dunn and Braun 1986b). The initial
selection for sagebrush at brood rearing sites may be linked to a female’s choice of nest
sites, as sagebrush is a relatively minor component of the diet of nesting females and
juveniles ≤ 10 weeks of age (Patterson 1952, Klebenow and Gray 1968, Peterson 1970).
Few studies have assessed whether habitat selection by Sage Grouse is based on a
minimum patch size of certain vegetation characteristics. Dunn and Braun (1986b)
60
measured horizontal cover 5 and 10 m from the center of summer use sites. They found
that the extent of horizontal cover at 5 m, but not 10 m, contributed to statistically
differentiating between summer use versus random sites. Data from telemetry studies
indicate that Sage Grouse select for certain vegetation characteristics at nest sites and
brood sites (Patterson 1952, Peterson 1970, Wallestad and Pyrah 1974, Schoenberg 1982,
Dunn and Braun 1986b, Drut et al. 1994a, Gregg et al. 1994, Sveum et al. 1998a, Sveum
et al.1998b). However, no attempt has been made to address the scale at which selection
is taking place. Nest success and brood survival should be related to the scale at which a
female selects habitat patches, which implies that females select nest and brood rearing
locations based on vegetation characteristics of a certain patch size.
The purpose of this study was to ascertain if Sage Grouse are selecting nest
locations and brood locations based on vegetation characteristics, and if there are certain
scales at which they are selecting habitat. I tested the null hypotheses that there were no
differences in vegetation characteristics between successful and unsuccessful nests, and
that there were no differences between nest locations and random sites, or brood locations
and random sites. I also tested the null hypothesis that Sage Grouse were not selecting
nesting or brood rearing locations based on a patch size of 7.5 and/or 15 m radius
surrounding nests and brood sites.
METHODS AND MATERIALS
I monitored the habitat selection of Sage Grouse within a 4,000 km2 area of
southeastern Alberta (49° 35’ N, 110° 50’ W). Silver sagebrush (A. cana) is the
dominant shrub and pasture sage (A. frigida) the dominant forb (Madsen 1995a, Aldridge
61
1998). Common grasses include speargrass (Stipa comata), june grass (Koeleria
macrantha), blue grama (Bouteloua gracilis) and western wheatgrass (Agropyron
smithii).
I captured females at 6 of 8 known active mating grounds (leks) from March
through May 1998 and 1999 using walk-in traps (Schroeder and Braun 1991) or with a
long handled hoop net and handheld spotlight (Giesen et al. 1982). Several females were
captured by nightlighting flocks of broodless females during summer. Sex and age
[(yearlings < 2 years old) and (adults ≥ 2 years old)] of all captured individuals were
assigned based on the shape and length of the outermost primaries (Eng 1955, Crunden
1963, Braun 2000). Captured females were fitted with a 14 g necklace style radio
transmitter (RI-2B transmitters; Holohil System Ltd. Carp, Ontario).
I used a 3-element Yagi antenna and portable receiver (TR2 scanning receiver,
Telonics Inc., Mesa, Arizona; Merlin 12 receiver, Custom Electronics of Urbana, Inc.,
Urbana, Illinois) to locate females every other day during the nesting period (Musil et al.
1994, Schroeder 1997). Locations were recorded in Universal Transverse Mercator
Coordinates using a hand held 12 Channel Global Positioning System (Garmin 12 XL
and GPS II Plus units; Garmin International Inc., Olathe, Kansas). When signals
disappeared, I searched the study area from a fixed-winged aircraft.
When approaching a nest, signals were triangulated until the marked bird could be
observed from approximately 30 m to minimize disturbance (after Schroeder 1997). Nest
site characteristics were measured similar to Klebenow (1969), Wallestad and Pyrah
(1974), Musil et al. (1994) and Commons (1997). At each nest site, I estimated the
percent sagebrush canopy cover, as well as the percent cover of grasses, palatable (to
62
Sage Grouse) forbs, non-palatable forbs, other shrubs, and bare ground/dead materials
within a 1 m2 quadrat using a protocol similar to Daubenmire’s (1959) method. The
mean maximum heights of the vegetation types were also calculated for each plot using
three measurements for each vegetation class. To identify the scale at which habitat
characteristics might be selected, I took measurements at the nest itself as well as at 8
additional dependent non-random 1 m2 plots. The additional plots were placed 7.5 and
15 m away from the nest site in each of the four ordinal directions. I used a similar
method to Canfield’s (1941) line intercept method to estimate the canopy cover of live
sagebrush along the four 15 m transects radiating from the nest site in each ordinal
direction. I estimated the density of sagebrush by counting the number of plants within
0.5 m of the transect (Commons 1997). The mean height of sagebrush along these
transects was also estimated. Measurements were recorded separately for the first and
second half (7.5 m each) of the transect.
Measurements of habitat characteristics were also taken at a dependent random
site using the same protocol for plots and line transects. I chose the random location by
walking 100 to 500 m (distance randomly chosen) in a random direction from the nest
site (dependent random plots). The closest sagebrush plant to the random location was
used as the “random” nest site, since the majority of nests are under sagebrush (Patterson
1952, Klebenow 1969, Connelly et al. 1991, Sveum et al. 1998b). The dependent nonrandom plots represented non-nest site characteristics within the same sagebrush “stand”,
and the dependent random plots represented non-nest site characteristics from different
stands.
63
I considered a nest successful if the amniotic membranes of one or more egg
shells in the nest were detached (Schroeder 1997). A nest was considered unsuccessful if
eggs were broken, or the membranes of egg shells remained intact. Vegetation
characteristics were measured at nest and random locations immediately following a
successful hatch, or after a predation event.
Broods were located regularly using telemetry; typically I attempted to get within
100 m of females and their broods once per week. These locations were used for habitat
analyses. I tried not to flush females and their broods at an early age (≤ 3 weeks of age).
Older broods were intentionally flushed once per week to estimate chick survival. Brood
“use” locations were recorded in Universal Transverse Mercator Coordinates (UTMs),
and I returned the following day to measure vegetation characteristics. I performed the
same vegetation measurements at brood use locations and corresponding dependent
random locations that I performed for nests.
Most vegetation variables had non-normal distributions and the means of some
variables were somewhat correlated with the variances, therefore, I log transformed all
variables. However, for reporting purposes, I present means and standard errors of the
un-transformed data.
I used forward stepwise Discriminant Functions Analysis (DFA) to determine if a
linear function of one or more variables discriminated between successful and
unsuccessful nest locations. I also used DFA to determine if early and late brood rearing
periods could be differentiated based on vegetation characteristics. I used a multivariate
paired T-test (T2 ; Morrison 1990) to test for differences between vegetation
characteristics at use (nests and broods) and random locations. I chose a paired design to
64
account for individual micro-site differences, and because my random locations were
dependent on use site locations. I chose a multivariate test which allowed me to
investigate potential interactions between variables and identify overall differences
between habitat at use and random locations. When the overall model was significant, I
used a post-hoc limits test of confidence intervals to test for differences between nests or
brood locations and random locations. If the 95% confidence interval difference for the
tested variable did not include zero, the variable was considered to make a significant
contribution to the model.
All analyses were conducted at four different scales. First, a test was completed
using only the 1 m2 plot at the use “site” (nest or brood) to differentiate between
vegetation characteristics at use sites alone (successful vs. unsuccessful nests and nest or
brood use sites versus random sites). I analysed vegetation characteristics for
nesting/brood rearing “areas” by averaging the measurements from all nine plots for each
use and random location. I analysed vegetation characteristics at two intermediate scales
by averaging measurements from the four plots at the 7.5 m radius and the four plots at
the 15 m radius (successful vs. unsuccessful nests and nest/brood versus random). For all
analyses, results were considered significant when alpha was < 0.05. Where necessary, I
applied a Bonferroni correction (Sokal and Rolf 1995).
RESULTS
I captured and fitted 7 females (4 adults and 3 yearlings) with transmitters in 1998
and 34 females (23 adults and 11 yearlings) in 1999. Due to predation events, difficulties
with two transmitters, the unknown fate of some females, and the fact that some females
65
were captured after the breeding season, data were collected for 22 females over
individual breeding seasons.
I had eleven habitat variables (Table 3.1) from plot measurements available to
enter into all models. I chose to enter only cover types into each model, as some
variables were highly correlated with height (R > 0.70, using a correlation matrix for all
use and random habitat measurements combined), indicating these variables measured
similar habitat characteristics (Table 3.1). Cover and height measurements for shrubs (R
= 0.928) and unpalatable forbs (R = 0.810) were correlated and measured the same
characteristics (Table 3.1). While the correlation between cover and height for other
vegetation measurements was not as strong (R < 0.70), each cover type was somewhat
correlated with height measurements of the same class (P ≤ 0.01, Table 3.1). Therefore, I
chose to enter only selected cover types and not vegetation heights into my models. Bare
ground was highly correlated with grass cover only (R > 0.70, Table 3.1) and was
inversely correlated to the sum of all cover types, thus, it was not entered into any
models. Unpalatable forbs represented a small percentage of cover at all habitat sites
(use and random combined; 0 = 1.1 ± 0.06) and this variable was not included in any
models. Thus, the variables that I entered into all models were sagebrush cover, shrub
cover, palatable forb cover, and grass cover. I did not incorporate line transect data into
models, as they were measured at different scales. Measurements of both sagebrush
density and line intercept of sagebrush along transects were both highly correlated with
sagebrush cover estimates from plots (R = 0.811, Table 3.1) and thus reflected
biologically similar characteristics. I analysed transect data individually using univariate
statistics to compare sagebrush characteristics.
66
Table 3.1. Spearman rank correlations for vegetation variables measured at all Sage
Grouse nest, brood, and random locations combined. Data for plot variables were
averaged for all nine plots, and data for line transects were averaged for all four
15 m transects. Using a Bonferroni Correction, " = 0.003.
R
t-value
P value
0.597
11.49
≤ 0.0001
Shrub Cover * Height
0.928
38.40
≤ 0.0001
Unpalatable Forb Cover * Height
0.810
21.30
≤ 0.0001
Palatable Forb Cover * Height
0.539
9.88
≤ 0.0001
Grass Cover * Height
0.209
3.30
≤ 0.001
Bare/Dead * Sagebrush Cover
-0.070
-1.08
0.28
Bare/Dead * Shrub Cover
-0.144
-2.24
0.03
Bare/Dead * Unpalatable Forb Height
-0.141
-2.19
0.03
Bare/Dead * Palatable Forb Cover
-0.307
-4.98
≤ 0.0001
Bare/Dead * Grass Cover
-0.792
-19.98
≤ 0.0001
Sagebrush Cover * Line Intercept - Cover
0.906
32.94
≤ 0.0001
Sagebrush Cover * Line Transect Height
0.472
8.24
≤ 0.0001
Sagebrush Cover * Line Transect Density
0.811
21.37
≤ 0.0001
Sagebrush Height * Line Transect Height
0.742
17.04
≤ 0.0001
PLOT VARIABLES
Sagebrush Cover * Height
n=240
PLOT VARIABLES and DEAD/BARE
SAGEBRUSH PLOTS and LINE TRANSECTS
67
Nests
In 1998, three nests of radio-marked birds were located and two additional nests
of unmarked females were found. In 1999, I located 24 nests used by radio-marked birds,
for a total of 29 nests. Due to the small number of nests located in 1998, I did not
statistically test for between year differences in vegetation characteristics. Yearlings
made four nests, adults made 23 nests, and two nests were of by unmarked females of
unknown age.
Vegetation characteristics were measured at all 29 nest locations (14 successful
and 15 unsuccessful nests) and 29 dependent random locations. Nest success in my study
(percent of all nests that hatched ≥ 1 egg including renesting attempts) was 46.2% for 26
nests; 1/3 nests in 1998, 11/23 nests in 1999 (Chapter 2). Three nests were not included
in this nest success estimate due to difficulties with one transmitter, and the biases
inherent in locating successful versus unsuccessful nests of unmarked birds.
Twenty-six of 29 (89.6%) nests were located under silver sagebrush (A. cana).
One of the three nests not under sagebrush was under snowberry (Symphoricarpos albus),
one was in tall grass along an irrigation dike (although there were some dead shrubs
present of an unidentified non-native Artemisia species), and the other nest was under an
uprooted tumbleweed (Salsola kali) plant in a wheat stubble field. Two of 26 nests under
sagebrush had ≤ 15% cover of sagebrush, but greater than 50% canopy cover of other
shrubs (snowberry and rose, Rosa spp.).
68
Nesting Habitat Characteristics by Nest Fate
I incorporated grass height into the DFA model to test for differences between
successful and unsuccessful nests only. Grass cover was only weakly correlated with
grass height (R ≤ 0.209, Table 3.1) and in two artificial nest predation experiments in the
study area, grass height was shown to be an important variable predicting nest fate (Seida
1998, Watters 1999). At nest “sites” (1 m2 plot over nest), successful nests had greater
sagebrush cover, shrub cover, and forb cover but lower grass cover. Vegetation variables
in all five-cover types were taller at successful nests (Table 3.2). Sagebrush cover was
the dominant cover type both at successful (32.9% ± 7.05) and unsuccessful nests (31.0%
± 4.58; Table 3.2). However, the DFA contrasting successful nests and unsuccessful
nests was not statistically significant (F4, 24 = 2.69, P > 0.068).
Overall (all 9 plots combined), successful nesting areas could be statistically
differentiated from the areas surrounding unsuccessful nests, as they had taller grass but
less grass cover (F2, 26 = 6.17, P < 0.006; Table 3.2). The DFA allowed for correct
classification 75.9% of the time (Table 3.3). Successful nests were at the positive end of
the discriminant function axis and unsuccessful nests at the negative end for all scales
tested (1, 7.5, 15, m and over all 9 plots). Grass cover [Wilks’ 8 = 0.937, P < 0.004,
Standardized Canonical Coefficient (SCC) = -1.039] contributed the most to the
discriminant function. Grass height (Wilks’ 8 = 0.830, P = 0.023, SCC = 0.845) was also
significant in the model (Table 3.3). Thus, at successful nests, there was less grass cover,
but taller grass (Table 3.2). The squared Mahalanobis distance (a measure of the distance
between two centroids on a linear axis) between successful and unsuccessful nest
locations (1.90) was significant (F2, 26 = 6.17, P < 0.0064).
69
Table 3.2. Vegetation characteristics at successful and unsuccessful nests. Values are shown as means ± one standard error.
Variables with a (*) were entered into the Discriminant Functions Analysis. Means with a (*) contributed significantly to
discriminating between successful (n=14) and unsuccessful (n=15) nests for that scale (P < 0.05, Table 3.3).
Nest Site
Variables
COVER (%)
Sagebrush Cover*
Shrub Cover*
Unpalatable Forb Cover
Palatable Forb Cover*
Grass Cover*
Dead/Bare Ground
HEIGHT (cm)
Sagebrush Height
Shrub Height
Unpalatable Forb Height
Palatable Forb Height
Grass Height*
7.5 m Plots
15 m Plots
All 9 Plots
Successful
Unsuccessful
Successful
Unsuccessful
Successful
Unsuccessful
Successful
Unsuccessful
32.9
(7.05)
8.9
(4.89)
0.7
(0.49)
10.4
(2.06)
26.8
(5.61)
20.4
(6.08)
31.0
(4.58)
6.0
(5.65)
0.0
7.9
(1.57)
2.2
(1.5)
0.4
(0.20)
13.9
(2.75)
33.2*
(5.80)
42.4
(6.51)
7.3
(1.7)
4.3
(3.46)
1.4
(0.49)
7.0
(1.03)
49.7*
(3.78)
30.3
(4.46)
8.5
(2.50)
1.4
(0.69)
0.4
(0.28)
10.3
(2.11)
36.2*
(5.32)
43.3
(6.89)
6.3
(1.43)
1.9
(1.04)
0.8
(0.26)
8.7
(1.20)
51.3*
(4.51)
31.1
(4.47)
10.9
(2.03)
2.6
(1.16)
0.4
(0.24)
11.9
(2.09)
33.8*
(5.36)
40.4
(6.47)
9.5
(1.57)
3.4
(2.6)
1.0
(0.30)
7.6
(0.84)
48.9*
(3.60)
29.5
(3.93)
42.0
(6.69)
12.1
(5.73)
0.9
(0.63)
20.1
(3.85)
37.0
(6.53)
40.7
(4.04)
5.4
(3.89)
0.0
18.6
(2.98)
5.9
(2.41)
0.6
(0.33)
17.4*
(2.91)
29.9*
(3.61)
22.9
(3.88)
10.5
(4.28)
4.3
(1.16)
10.1*
(1.73)
24.8*
(2.60)
20.2
(4.31)
6.1
(2.91)
0.4
(0.27)
17.0
(2.75)
32.8*
(3.98)
26.4
(4.48)
12.2
(5.72)
3.8
(1.13)
10.7
(1.22)
23.0*
(2.26)
24.4
(4.31)
15.2
(4.46)
1.1
(0.52)
18.5
(2.45)
31.6*
(4.06)
28.5
(3.43)
16.1
(5.90)
4.9
(1.1)
10.8
(1.24)
24.4*
(2.25)
6.0
(0.72)
36.7
(5.47)
20.3
(6.01)
11.2
(1.88)
25.3
(2.83)
70
The DFA at the 7.5 m scale (4 plots at 7.5 m) was significant (F2, 26 = 3.77, P <
0.023) and correctly classified nest fate 72.4% of the time (Table 3.3). Grass cover,
which was greater at unsuccessful nesting areas at 7.5 m, contributed the most to the
discriminant function (Wilks’ 8 = 0.908, P < 0.009, SCC = -0.926). Palatable forb cover
(Wilks’ 8 = 0.740, P < 0.077, SCC = 0.494) and grass height (Wilks’ 8 = 0.725, P <
0.260, SCC = 0.430) made smaller but significant contributions to the discriminant
function and were both greater at 7.5 m surrounding successful nests. The squared
Mahalanobis distance between successful and unsuccessful nest locations (1.81) was
significant (F2, 26 = 3.77, P < 0.0232; Table 3.3).
At the 15 m scale (4 plots at 15 m), successful nest locations also had less grass
cover but significantly taller grass (Table 3.2). The DFA was significant (F2, 26 = 8.056,
P < 0.005) and allowed for correct classification of nest locations 82.8% of the time.
Grass cover again contributed the most to the discriminant function (Wilks’ 8 = 0.861, P
< 0.003, SCC = - 0.979). Grass height also significantly contributed to the model (Wilks’
8 = 0.862, P ≤ 0.003, SCC = 0.980). The squared Mahalanobis distance between the
groups (2.48) was significant (F2, 26 = 8.05, P < 0.005; Table 3.3).
Nest Habitat
Nests could be significantly differentiated from random sites by vegetation
characteristics (T2 = 18.35, P ≤ 0.001; Table 3.4). A limits test on the 95% confidence
intervals indicated that sagebrush cover was the only variable that contributed to the
difference between nests (1 m plot) and random sites. Sagebrush cover was the greatest
71
Table 3.3. Discriminant Functions Analysis of vegetation characteristics at successful and unsuccessful nests. Values are shown as
means ± one standard error. Sagebrush cover, shrub cover, palatable forb cover, grass cover, and grass height were the only
variables entered into the model. The models for the 7.5 m, 15 m, and all 9 plot scales were all significant (P < 0.05).
Variables that contributed significantly to discriminating between successful (n = 14) and unsuccessful (n = 15) nests for that
scale are shown below (P < 0.05). The larger the Wilks’ 8, the greater the variable contributed to the discriminant function.
SCC = Standardized Canonical Coefficient.
Over all Model
All 9 Plots
15 m Plots
7.5 m Plots
Wilks’ 8
0.678
0.617
0.688
F
6.17
8.056
3.772
0.0064
0.0016
0.023
75.9
82.8
72.4
P
% Correct
Classification
Variables
Wilks’
8
0.937
P-level
R2
SCC
0.004
0.204
Palatable
Forb Cover
/
/
Grass Height
0.830
0.023
Grass Cover
P-level
R2
SCC
- 1.039
Wilks’
8
0.861
0.003
0.223
/
/
/
/
0.204
0.845
0.862
0.003
72
P-level
R2
SCC
- 0.979
Wilks’
8
0.908
0.009
0.094
- 0.926
/
/
0.740
0.182
0.077
0.494
0.230
0.980
0.725
0.260
0.123
0.430
Table 3.4. Vegetation characteristics at nests and random locations. Values are shown as means ± one standard error. Variables with
a (*) were entered into the model. Means with a (*) contributed significantly to differentiating between nest locations and
random locations for that scale. (n = 29 for nests and random locations at all scales).
Nest Site
Variables
COVER (%)
Sagebrush Cover*
Shrub Cover*
Unpalatable Forb Cover
Palatable Forb Cover*
Grass Cover*
Dead/Bare Ground
HEIGHT (cm)
Sagebrush Height
Shrub Height
Unpalatable Forb Height
Palatable Forb Height
Grass Height
7.5 m Plots
15 m Plots
All 9 Plots
Nest
Random
Nest
Random
Nest
Random
Nest
Random
31.9*
(4.07)
7.4
(3.70)
0.3
(0.24)
8.1
(1.12)
31.9
(3.96)
20.34
(4.20)
15.7*
(2.44)
1.7
(1.39)
0.2
(0.2)
8.4
(1.03)
41.7
(4.83)
32.2
(4.01)
7.6*
(1.14)
3.3
(1.91)
0.9
(0.28)
10.3
(1.55)
41.7
(3.70)
36.1
(4.00)
3.6*
(0.86)
1.1
(0.41)
1.1
(0.25)
10.6
(1.53)
41.7
(4.04)
41.9
(4.17)
7.3
(1.41)
1.67
(0.62)
0.6
(0.19)
9.4
(1.18)
44.0
(3.70)
37.0
(4.14)
4.9
(1.04)
1.4
(0.41)
1.2
(0.35)
11.9
(1.58)
42.9
(3.38)
37.7
(3.21)
10.2
(1.26)
3.1
(1.43)
0.7
(0.20)
9.7
(1.15)
41.6
(3.44)
34.8
(3.80)
5.5
(0.77)
1.3
(0.36)
1.1
(0.26)
10.9
(1.39)
42.2
(3.6)
39.0
(3.64)
41.3
(3.78)
8.6
(3.42)
0.4
(0.31)
15.5
(2.22)
30.9
(3.58)
27.5
(3.38)
2.2
(1.35)
0.1
(0.07)
11.2
(1.24)
28.5
(1.97)
20.8
(2.46)
8.3
(2.50)
2.5
(0.70)
13.6
(1.78)
27.2
(2.21)
14.4
(2.10)
5.7
(2.89)
2.3
(0.56)
12.5
(1.11)
25.7
(1.92)
23.4
(3.11)
9.3
(3.27)
2.2
(0.67)
13.8
(1.56)
27.7
(2.39)
16.3
(2.19)
10.0
(3.75)
1.7
(0.43)
13.6
(1.15)
27.0
(1.84)
26.5
(2.56)
15.7
(3.67)
3.0
(0.71)
14.5
(1.51)
27.9
(2.34)
22.2
(1.77)
10.9
(3.53)
2.4
(0.47)
12.9
(10.7)
26.6
(1.80)
73
cover type at nest sites (31.9% ± 4.07) where it was more than double that of random
sites (15.7% ± 2.4 4; Table 3.4). Shrub cover was also greater at nest sites, although not
significantly so. None of the vegetation height measurements was entered into the model
due to correlations with cover. Sagebrush height and grass height were both greater at
nests than random sites; however, not significantly (P ≥ 0.01; univariate paired t-test
using a Bonferroni correction factor). Within nesting areas, nests were placed under tall
sagebrush plants, which averaged 41.3 cm ± 3.78 in height (Table 3. 4).
Nesting areas could be significantly differentiated from dependent random
locations at the 7.5 m scale (T2 = 32.2, P ≤ 0.001). Sagebrush cover was the only
variable that contributed significantly to the model and was greater at nest locations
(7.6% ± 1.14 vs. 3.6% ± 0.86). A univariate paired t-test indicated that none of the height
measurements at the 7.5 m scale differed between nest and random locations (P ≥ 0.01,
using a Bonferroni correction factor), even though sagebrush and other shrubs were taller
at the 7.5 m scale at nest locations.
Nest locations could not be differentiated from random locations at the 15 m scale
(T2 = 5.64, = 1.26, P ≥ 0.05), or over the entire nesting area (all 9 Plots; T2 = 17.52, =
3.91, P ≥ 0.05). None of the five height variables at nest locations at the 15 m scale alone
or over all 9 plots combined differed from random locations (P ≥ 0.01, using a Bonferroni
correction factor).
I combined sagebrush and other shrub cover to determine if nest locations could
be differentiated from random locations on this basis. Overall shrub cover did allow
significant differentiation of nests from random locations at the nest site (T2 4, 25= 20.12, P
74
≤ 0.01), at the 7.5 m scale (T2 4, 25 = 17.62, P ≤ 0.01), and for all nine plots combined, (T2 4,
25
= 18.81, P ≤ 0.01), but not at the 15 m scale alone (T2 4, 25 = 4.94 P ≥ 0.05).
Brood Habitat
I gathered data on habitat use by 15 different radio-marked females and their
broods. Vegetation characteristics were measured at 91 brood locations (63 for broods <
7 weeks of age and 28 for broods 7 to 12 weeks of age) and 91 corresponding dependent
random locations. I entered sagebrush cover, shrub cover, forb cover, and grass cover
into a forward stepwise DFA, but could not discriminate between early (< 7 weeks of
age) and late brood (7 to 12 weeks of age) locations (P ≥ 0.05). Thus, I combined early
and late brood rearing locations to test for overall habitat selection (use vs. random
locations).
Brood use “sites” (one plot centered on the location where the marked female and
her brood were located) could be differentiated from random sites (T2 4, 87 = 155.07, P ≤
0.001; Table 3.5). Brood rearing areas could also be significantly differentiated from
random locations overall (all 9 m plots; T2 4, 87 = 72.06, P ≤ 0.001), and at both the 7.5 m
(T2 4, 87 = 21.48, P ≤ 0.001) and 15 m scales (T2 4, 87 = 28.79, P ≤ 0.001; Table 3.5). A
limits test indicated that sagebrush cover was the only variable entered into the model
that allowed for differentiation at all levels. Palatable forb cover was low at brood use
locations, ranging from 10.9 to 12.9% at the different scales (Table 3.5). Likewise, none
of the vegetation heights was entered into the model due to correlations with cover
measurements. However, the heights of all vegetation types at brood sites and brood
rearing areas (7.5 m scale, 15 m scale, and overall) were greater than at random
75
Table 3.5. Vegetation characteristics at brood use and random locations. Values are shown as means ± one standard error. Variables
with a (*) were entered into the model. Means with a (*) contributed significantly to differentiating between brood locations
and random locations for that scale.
Brood Site
COVER (%)
Sagebrush Cover*
Shrub Cover*
Unpalatable Forb Cover
Palatable Forb Cover*
Grass Cover*
Dead/Bare Ground
HEIGHT (cm)
Sagebrush Height
Shrub Height
Unpalatable Forb Height
Palatable Forb Height
Grass Height
7.5 m Plots
15 m Plots
All 9 Plots
Brood
Random
Brood
Random
Brood
Random
Brood
Random
20.9*
(1.63)
1.2
(0.39)
0.9
(0.23)
10.9
(1.20)
34.2
(2.05)
32.0
(2.07)
2.9*
(0.54)
1.7
(0.92)
1.4
(0.36)
11.5
(1.52)
36.9
(2.52)
45.5
(2.72)
7.1*
(0.68)
1.6
(0.34)
1.1
(0.20)
12.8
(1.21)
36.0
(1.71)
41.5
(1.73)
4.7*
(0.50)
1.9
(0.53)
1.4
(0.25)
11.5
(1.23)
33.8
(1.84)
46.7
(2.14)
7.3*
(0.69)
2.0
(0.53)
1.1
(0.19)
12.9
(1.07)
35.2
(1.75)
41.6
(1.76)
4.6*
(5.52)
2.1
(0.62)
1.3
(0.21)
10.7
(1.06)
36.6
(1.98)
44.7
(2.07)
8.7*
(0.65)
1.7
(0.36)
1.1
(0.15)
12.6
(1.10)
35.4
(1.61)
40.5
(1.65)
4.5*
(0.47)
2.0
(0.57)
1.3
(0.21)
11.2
(1.14)
35.4
(1.89)
45.7
(2.06)
32.0
(2.36)
2.5
(0.83
1.4
(0.50)
21.7
(1.46)
45.3
(1.86)
7.6
(1.2)
2.2
(0.89)
1.2
(0.51)
15.3
(1.37)
36.6
(2.01)
22.9
(1.61)
6.7
(1.22)
2.9
(0.42)
20.7
(1.16)
41.5
(1.36)
16.0
(1.52)
5.3
(11.78)
2.2
(0.35)
17.5
(1.13)
39.1
(1.72)
21.9
(1.48)
7.0
(1.29)
2.3
(0.39)
20.4
(1.07)
43.1
(1.33)
15.5
(1.34)
6.4
(1.34)
2.0
(0.32)
17.5
(1.18)
39.7
(1.50)
27.6
(1.61)
8.8
(1.35)
4.1
(0.57)
20.8
(1.05)
42.5
(1.30)
17.8
(1.30)
6.9
(1.29)
2.8
(0.40)
17.8
(1.11)
39.5
(1.53)
76
sites/locations (Table 3.5). Only the height of sagebrush and palatable forbs were
significantly greater at brood use sites/locations for all scales (P ≤ 0.01, using a
Bonferroni correction factor). Grass was significantly taller only at the brood site level
(P ≤ 0.01). Grass height at brood locations was similar at all scales, ranging from 41.5 to
45.3 cm, and was taller than grass height at nest locations (range 27.2-30.9; Table 3.4).
Line Transects
I used the line intercept method (Canfield 1941) to estimate sagebrush canopy
cover and line transects to estimate sagebrush vegetation characteristics (height and
density). Since sagebrush was the only variable that differentiated between use and
random locations, I compared line intercept and line transect data to assess which
characteristics of sagebrush were selected for by Sage Grouse (Table 3.6).
At nesting locations, sagebrush cover was greater over the entire 15 m radius surrounding
nest sites, when compared to random locations. Cover was also greater at nesting areas
than random locations at 0 to 7.5 m from the nest and between 7.5 and 15 m from the
nest. However, the estimate of sagebrush cover at nest locations using the line intercept
method (all 15 m; 4.5% ± 0.65) was significantly less than that using all nine plots
(10.9% ± 1.26; t 28 = 8.93, P ≤ 0.001; Table 3.4).
Sagebrush density over the entire 15 m radius was greater surrounding nest sites,
than random sites (P ≤ 0.017, using a Bonferroni correction factor), however, when
separated into the two scales, density was only greater ≤ 7.5 m from nest sites (P ≤ 0.017)
and not from 7.5 to 15 m (P ≥ 0.017). Sagebrush height along line transects was not
significantly different between nest and random locations at all scales (P ≥ 0.017,
77
Table 3.6. Sagebrush characteristics at use locations (nest and brood) and random locations along line transects. Values are shown as
means ± one standard error. Means with a (*) were significantly different between use locations and random locations for that
scale using a univariate paired t-test. A Bonferronni Correction was used (% = 0.017).
≤ 7.5 m
Variables
7.5 m to 15 m
All 15 m of Transect
Use
Random
Use
Random
Use
Random
NESTS (n = 29)
Sagebrush Cover (%)
Line Intercept
Sagebrush Height
(cm)
Sagebrush Density
(# Plants/m2 )
5.6*
(0.75)
26.4
(2.50)
2.1*
(0.28)
2.9*
(0.52)
19.9
(1.59)
1.5*
(0.36)
3.5*
(0.64)
22.1
(2.54)
1.7
(0.24)
1.8*
(0.51)
17.1
(1.31)
1.4
(0.30)
4.5*
(0.65)
24.4
(2.45)
1.9*
(0.25)
2.4*
(0.48)
18.5
(1.29)
1.4*
(0.32)
BROODS (n = 91)
Sagebrush Cover (%)
Line Intercept
Sagebrush Height
(cm)
Sagebrush Density
(# Plants/m2 )
5.8*
(0.47)
26.0*
(1.60)
1.9*
(0.19)
2.4*
(0.30)
19.0*
(1.57)
1.2*
(0.16)
4.3*
(0.42)
24.2*
(1.42)
1.8*
(0.17)
2.7*
(0.36)
17.6*
(1.30)
1.2*
(0.14)
5.0*
(0.42)
25.6*
(1.45)
1.8*
(0.17)
2.5*
(0.31)
19.6*
(12.7)
1.2*
(0.14)
78
Table 3.6). Sagebrush height (24.4 cm ± 2.45 for the entire nesting area along transects)
was not significantly different from estimates using all 9 plots (26.5 cm ± 2.56; t 28 =
2.03, P ≥ 0.05).
Using line transect data, brood use areas had taller sagebrush with more canopy
cover and a greater density of sagebrush than random locations at all three scales (≤ 7.5
m from brood sites, 7.5 to 15 m from sites, and over all 15 m; P ≤ 0.017; Table 3.6).
However, as was the case at nest locations, values for sagebrush cover at brood locations
(all 15 m intercept; 5.0% ± 0.42; Table 3.6) were significantly less than values estimated
using plots (8.7% ± 0.65; t 90 = 12.94, P ≤ 0.001; Table 3.5). Sagebrush height estimated
at brood locations from line transects (25.6 cm ± 1.45; Table 3.6) was similar to height
estimated from plots (27.6 cm ± 1.61; t 90 = 1.81, P ≥ 0.05; Table 3.5).
DISCUSSION
Although Sveum et al. (1998b) found no difference in the probability of success
for nests under sagebrush compared to nests under other plants, Connelly et al. (1991)
found that nests under non-sagebrush plants were less successful (P ≤ 0.025). In the
Connelly et al. (1991) study, 21% of nests (18/84) were under species other than live
sagebrush, even though sagebrush comprised ≥ 16% of the available canopy cover in the
area. This suggests that Sage Grouse may be selecting nest sites based on suitable
amounts of shrub and herbaceous cover regardless of the vegetation species that provides
it (Connelly et al. 1991, Sveum et al. 1998b).
The majority of the nests I found (89.6%, 26 of 29) were under sagebrush, even
though it comprised ≤ 11% of the canopy cover at nest locations and ≤ 6% of the canopy
79
cover available in the area (Table 3.4). Thus, there is less available sagebrush habitat in
Canada than in other areas. This is likely due to the species of sagebrush present.
Artemisia cana is the only shrubby species of tall sagebrush available to Sage Grouse in
Canada. It is smaller and does not provide as much cover as big sagebrush (A. tridentata
spp.) found throughout most of the core range of Sage Grouse. Despite these differences,
nest success in my study (46.2%) was comparable to other areas (30 to 60%; Schroeder et
al. 1999).
Successful nests could not be differentiated from unsuccessful nests by the
vegetation characteristics of the nest site itself. In Alberta, Seida (1998) found that
artificial Sage Grouse nests were more likely to be successful if they had taller grass and
more forb cover at the nest site. In a similar experiment, Watters (2000) found that
successful nests had taller forbs and grass, but less grass cover and shorter sagebrush
surrounding the nest. These results are similar to those of both natural (Gregg et al. 1994,
Sveum at al. 1998a) and artificial nests (Delong et al. 1995) at other locations. This
highlights the importance of greater cover of medium height sagebrush and tall grasses.
Watters (2000) found that successful nests had less grass cover, similar to my results for
natural nests, which contrasts with other studies. This difference is not easily explained,
but may be related to grazing. Perhaps more importantly, grass height was vital to nest
success in both instances. Nests located in areas that are intensely grazed will intuitively
have shorter grasses, resulting in decreased nest success. However, continual grazing
may cause the grasses to tiller (fill out and add stems) and increase the grass cover at
unsuccessful nests.
80
Most nests are found under sagebrush (Patterson 1952, Klebenow 1969, Connelly
et al. 1991, Gregg at al. 1994, Sveum et al. 1998b), with selection for taller plants that
generally provide greater amounts of cover (Wallestad and Pyrah 1974, Musil et al.
1994). I found that females selected nest locations that had a greater amount of
sagebrush cover compared to available cover at the nest site itself, and up to 7.5 m from
nests. However, sagebrush characteristics were not selected for at the 15 m scale. Sage
Grouse are selecting nesting areas based on sagebrush stands that are at least 7.5 m, but
not 15 m in radius, which provide greater amounts of cover (7.6% ± 1.14).
Sage Grouse selected nest areas that had a greater density of sagebrush within 7.5
m of nest sites. There was no difference in sagebrush density from 7.5 to 15 m from nest
sites. Within the stands selected, females placed their nests under taller sagebrush (0 =
41.3 cm ± 3.78) that provide greater canopy cover (0 = 31.9% ± 4.07). Sveum et al.
(1998b) found that Sage Grouse nested in locations based on vegetative characteristics at
both the nest-area and nest-site levels, similar to my study. However, they did not test for
vegetation differences at different distances from nest sites and were unable to compare
vegetation characteristics between nest sites and nest areas because of differences in
measurements scales. To my knowledge, my study is the first to show that Sage Grouse
select nesting areas based on habitat characteristics of a certain patch size and that nest
sites are selected for within those patches.
When sagebrush and other shrub cover were combined in my analysis, total shrub
cover at nest sites was greater than random locations. Nest areas had greater total shrub
cover at the 7.5 m scale but there was no difference 15 m from nest sites. Shrub height
was highly correlated with shrub cover, and sagebrush height was weakly correlated with
81
sagebrush cover. Thus, a female’s selection of nest habitat appears to be based on
structure; shrubs that are taller and provide more cover in an area at least 7.5 m in radius
but less than 15 m in radius are selected for as nesting areas. The tallest shrubs providing
the greatest cover within those stands are typically used for nest sites. Other studies have
shown that Sage Grouse tend not to place nests under the tallest available sagebrush
(Klebenow 1969, Gregg et al. 1994, Sveum at al. 1998b). This is because tall shrubs are
often associated with reduced lateral cover, due to a depleted understory (Klebenow
1969). Since the canopy of A. cana is not as dense as A. tridentata spp., the understory
may not be as sheltered and, thus, is not as depleted. If Sage Grouse in Canada select the
tallest sagebrush available, compared to available habitat, the understory is still suitable.
Other shrubs may also provide suitable concealment of nests, but sagebrush is by far the
most common shrub in the area.
For both nest and brood locations, the line intercept method resulted in
significantly lower estimates of sagebrush canopy cover than estimates generated from
plots. While these two methods resulted in different estimates, they were highly
correlated (R = 0.906; Table 3.1), indicating that both techniques accurately reflect
relative sagebrush cover. However, in absolute terms, one or both of the measures are
not precise. Regardless of the method used, my estimates of sagebrush canopy cover
(plots 10.9%; line intercept 4.5%) are below the 20 to 50% recommended as suitable
nesting habitat (Patterson 1952, Klebenow 1969, Wallestad and Pyrah 1974, Braun et al.
1977, Schroeder et al. 1999). Despite the low amount of cover provided by sagebrush in
southeastern Alberta, differences in the amount of cover were great enough that birds
were able to select for areas and sites with greater sagebrush cover to nest.
82
Broods remained in areas with denser, taller sagebrush, which was comparable to
sagebrush characteristic of nest locations. Greater sagebrush cover differentiated brood
use from random locations at all measured scales. In contrast, Klebenow (1969) found
that brood rearing habitat in Idaho had a lower density of sagebrush than was available.
Dunn and Braun (1986b) also showed that hens with broods selected areas with less
sagebrush. My data are the first to suggest that brood rearing locations had greater
sagebrush cover than was available at random locations.
While brood sites and brood rearing areas had more sagebrush than random
locations, Sage Grouse appear to be selecting brood rearing locations on a larger scale
than that at which nesting areas were selected for (areas of radius greater than 15 m).
Dunn and Braun (1986b) found that summer habitat use locations for broods and
unsuccessful females had taller sagebrush compared to random locations, but there was
no difference in sagebrush height surrounding the use site itself. They also found that use
locations compared to random locations had greater horizontal cover at 5 m from use
sites, but not at 10 m, suggesting that scaled habitat selection may be occurring. Other
than horizontal cover, Dunn and Braun (1986) did not measure vegetation characteristics
> 5 m from use sites.
Grass cover at brood use locations (35.4% ± 1.61) was similar to nesting areas
(41.6% ± 3.44), however, grass was taller at brood locations (42.5 cm ± 1.30 vs. 27.9 cm
± 2.34). This could simply be due to increased growth later in the season. Sveum et al.
(1998a) found that broods selected areas with more protective cover and tall (≥ 18 cm)
grass cover and vertical vegetation cover. Grass cover available to broods in my study
averaged 39.5 cm (± 1.53) in height, reflecting the exceptional grass growth over the
83
course of my study. I conclude that suitable protective cover was not limited during
1998-99.
I did not observe a shift in habitat used by broods, which typically occurs due to
changing dietary requirements of chicks (Martin 1970, Peterson 1970, Johnson and
Boyce 1990, Drut et al. 1994a, 1994b). Early brood rearing locations tend to be in
sagebrush uplands, but as temperature increases and moisture disappears, forbs become
desiccated, and broods shift to more mesic sites that have increased availability of forbs
(Peterson 1970, Dunn and Braun 1986b, Sveum et al. 1998a). Sveum et al. (1998a)
suggested that broods remained in areas with more sagebrush during early brood rearing
for the added escape cover it provides. Broods may remain in sagebrush uplands during
years when above average precipitation results in increased forb availability (Dunn and
Braun 1986b). Spring precipitation was above average in both years of my study
(Chapters 2, 4), resulting in increased vegetation growth. Thus, broods likely had
increased food resources available to them in sagebrush habitat, allowing them to remain
in sagebrush uplands. In dry years, broods should have to move from sagebrush uplands
to more mesic sites. However, mesic wetland type habitats are generally limited and may
not be available for broods in dry years. Thus, brood survival may be even lower during
times of drought.
Forb cover in brood use areas averaged 12.6% for early and late brood rearing
periods combined. Schoenberg (1982) found that young broods in Colorado used areas
with relatively low forb cover (6.9%) and quickly moved to wet meadows where forbs
comprised 41.3% of the cover. Peterson (1970) also found that forb cover was important,
accounting for 33% of the available cover at brood use sites. Forb cover in Oregon was
84
estimated at 10-14% for early brood rearing locations and 19-27% for late brood rearing
locations (Drut et al. 1994a). Drut et al. (1994a) suggested that 12-14% forb cover might
represent the minimum cover needed for brood habitat.
Increased forb cover in years with above average spring precipitation (Peterson
1970) may allow for greater movements and distribution of broods (Dunn and Braun
1986b). In years when precipitation is average or below average, forb cover may be
below that required to provide suitable brood habitat. The lack of a shift in brood habitat
between early and late-brood rearing in my study, suggests that differences in the
availability of forbs did not exist, at least in wet years. However, the limited cover
provide by forbs at brood locations despite high spring precipitation suggests that key
brood habitat in moist wetlands and drainages may be limiting in southeastern Alberta,
even in moist years.
Nesting success (46.2%) was within the range reported for studies in other areas.
Five of 12 (41.7%) females that were successful nesters raised at least one chick to
fledging (50 days of age; Schroeder 1997). However, the percentage of chicks that
survived from hatch to fledge (50 days of age) was only between 14 and 23% (Chapter
2), which is extremely low, given that estimates of between 33 and 38% have been found
in slightly declining populations (June 1963, Schroeder 1997). Despite low chick
survival estimates, grass cover and height likely contributed to suitable escape cover
throughout the study area. However, the lack of cover from sagebrush may ultimately
make the available escape cover unacceptably low.
Virtually all research on brood habitat use has found that areas with forbs are
selected and that a shift to more mesic sites occurs after broods reach six weeks of age.
85
Sage Grouse broods in my study did not select use sites based on forb availability and no
shift in habitat use occurred. Mesic areas with nutrient rich forbs may be limiting and
forbs as a food resource may be even more limiting during drier years. Lack of suitable
moist drainages for broods to forage in may also be a factor contributing to low chick
survival and poor recruitment (Chapters 1 and 2).
Management strategies for Sage Grouse should consider the identification and
protection of sagebrush stands that are at least 7.5 m in radius and preferably 15 m.
These stands should also have a suitable understory of tall grasses and forbs to enhance
nest concealment. In Canada, sagebrush is also an important component of brood rearing
habitat. My results suggest that managing for suitable nesting areas will also provide
suitable brood rearing sites for Sage Grouse, at least in some years. Even though spring
precipitation was higher than normal over the course of this study, important mesic areas,
such as wet meadows that provide high quality succulent forbs of ≥ 12% cover, may be
lacking within the Canadian range of Sage Grouse. This decreased availability of food
resources for chicks may be related to reduced chick survival and overall low
recruitment. Additional mesic areas with higher forb availability should be identified and
protected to increase chick survival. Management strategies should focus on enhancing
the number and quality of mesic sites where increased forb growth can occur.
Considerations should be given to managing grazing within mesic habitats to decrease the
effects that cattle may have on reducing the amount of succulent forbs necessary for
chick survival.
86
CHAPTER 4.
MODELING POPULATION TRENDS
87
In Chapter 1, I discussed historical Sage Grouse population trends in Canada. I
gathered data on population parameters (Chapters 2 and 3), which allowed me to develop
a model to predict future population trends. The parameters that I measured may be
influenced by intrinsic factors such as density dependence or genetic heterogeneity, and
also extrinsic factors such as climate. In this Chapter, I will assess some of these
influences on previous population trends and integrate the parameters that I measured
into the model to predict future population trends.
The Alberta Sage Grouse population declined by 66-92% from historical levels, to
an estimated 210 and 311 individuals in 1994 (Chapter 1). Lek counts in both Alberta
and Saskatchewan show the Canadian population has remained relatively stable but at
low numbers since that time (Fig. 1.3). The reason(s) for the sharp decline from the
population maintained in the early to mid-1980s is/are not clearly understood, but might
be related to changes in habitat quality and recruitment (Chapters 2, 3). The population
appears to have stabilized from 1996 to 1999, although this may simply be a consequence
of intensified lek count efforts in both provinces over that time (Aldridge 1998; W. C.
Harris, pers. commun.). Although Sage Grouse were not heavily hunted in Alberta in the
early 1990s (K. J. Lungle pers. commun.), they have not been hunted at all since 1996. It
is therefore also possible that removal of hunting pressure in Alberta has allowed the
declining population to stabilize.
Although a number of factors may have resulted in the relatively stable population
over the last six years, I believe it is most likely due to increased spring precipitation.
Sage Grouse productivity appears to be positively correlated with increasing spring
(April to June) precipitation (June 1963, Gill 1966, Chapter 2). Years with below
88
average spring moisture result in less vegetation growth, likely reducing Sage Grouse
nest success, as well as limiting the availability of lush vegetation important as food for
chicks (Martin 1970; Peterson 1970; Johnson and Boyce 1990; Klott and Lindzey 1990;
Drut et al. 1994a, 1994b; Pyle and Crawford 1996; Sveum et al. 1998a). Spring
precipitation from 1994 to 1999 in southeastern Alberta was above average (Fig. 4.1)
and, thus, it is likely that Sage Grouse productivity was relatively high. When the
population was declining in the late 1980s, the prairies experienced some of the driest
springs over the previous 30 years (Fig. 4.1). Population data for Alberta suggest a
correlation between spring precipitation and the number of males counted on leks the
following year, although the trend is not statistically significant (P ≥ 0.05). The lack of
continuous lek count data decreased the statistical power to detect a significant trend. If
my hypothesis is true, I predict that when spring precipitation is below average,
productivity will be adversely affected and the population will decline.
Little is understood about the survival and life history requirements of Sage
Grouse chicks, due to the difficulty of finding them. Only 24 of 96 Sage Grouse captured
in 1998 and 1999 were yearlings (Chapter 1), suggesting that recruitment over the course
of my study was about 25%. Other long-term research on stable Sage Grouse
populations has found that approximately 50% of captured birds are yearlings (Dalke et
al. 1963, Eng 1963, Jenni and Hartzler 1978, Chapter 1). This suggests that recruitment
is low in Canada, possibly due to low chick survival and/or high overwinter mortality.
89
Alberta Males Counted
Spring Precipitation
Mean Spring Precipitation
700
600
Number
500
400
300
200
100
0
1965
1970
1975
1980
1985
1990
1995
2000
Figure 4.1. Maximum Sage Grouse lek counts in Alberta shown as a function of spring
(April - June) precipitation (mm) for each year. The mean spring precipitation
since 1928 is shown for reference. Years when sampling efforts consisted of less
than eight surveyed leks are not included.
Using the survival rates and measures of productivity that I calculated from my
results (Table 4.1), I developed a population model to predict the future Alberta/Canadian
Sage Grouse population. The model incorporates estimated survival rates from radiomarked males and females, clutch size, egg viability, breeding success (includes nesting
success, and renesting attempts), and chick survival (Table 4.1). Annual female survival
is likely not as high if overwinter mortalities are considered. However, survival estimates
for this population are also likely slightly underestimated due to the biases associated
with radiotelemetry studies (Chapter 2). I also assumed that juvenile overwinter survival
is 100%, which is unlikely. Female survival in Alberta (56.5% from March to August)
90
was within the expected range. To simplify the model, I did not incorporate any
stochastic measures. Thus, all parameters are fixed and do not vary between years when
the model is simulated, even though these will likely change from year to year. For any
given year (Nt ), the subsequent year’s population can be predicted as follows: Nt+1 =
ENt+1 + GNt+1 .
Where:GNt+1 = (N tG × Gsurv) + ((N t recruit )/2) and
ENt+1 = (N tE × EBsurv × E Wsurv) + ((N t recruit )/2) and
Nt recruit = NtE × EBsurv × EWsurv × ClSize × Htc × BrSucc × ChFlsurv × ChWsurv
See Table 4.1 below for an explanation of model parameters. Nt recruit is annual
recruitment.
Table 4.1. Parameters used in population model. Overwinter survival of females and
chicks was not measured and was set at 100%. Based on 1999 lek count data, low
spring population estimates of 140 males and 280 females and high estimates of
207 males and 415 females were used as starting population numbers for the
model.
Population Parameter
Variable
Measure
Annual Male Survival
Female Survival (breeding season to fall)
GAsurv
EBsurv
31.0%
56.5%
Female Overwinter Survival (assumed)
EWsurv
100%
Clutch Size
ClSize
Egg Viability
Breeding Success
Chick Survival to Fledge (50 days)
Chick Overwinter Survival (assumed)
Htc
BrSucc
ChFlsurv
ChWsurv
91
7.75 eggs/nest
92%
54.5%
18%
100%
To test whether the model reflects the actual population trend over the past 30
years in Alberta, I ran the model using the 1968 population estimate as the starting point.
I let the model run from 1968 to 1999 and compared predicted population numbers from
the model to the actual population estimate based on lek counts for each year (Fig. 4.2).
The actual population estimates fluctuate consistently around the model generated
population numbers. This suggests that the population parameters that I measured in
1998 and 1999 are generally representative of the population over the last 30 years.
3000
Low Estimate
High Estimate
Low Predicted
High Predicted
Number of Birds
2500
2000
1500
1000
500
0
1965
1970
1975
1980
1985
1990
1995
Year
Figure 4.2. Actual Sage Grouse population size in Alberta based on lek counts for 1968
through 1999 shown with the predicted population superimposed. Predicted
population size is based on survival and productivity data measured in 1998 and
1999. The model starting point is based on the population in 1968. Years when
sampling efforts consisted of less than eight leks surveyed are not included. Lines
are drawn to illustrate trends between years with consecutive lek counts, and to
show the predicted population trend.
92
I iterated the population model 31 years into the future, from 2000 to 2030, to
examine future Sage Grouse population status for Alberta (Fig. 4.3). I used both high and
low population estimates in 1999 as starting points. From these data, I predict the
Alberta spring population will decrease from between 420 and 622 individuals in 1999 to
between 397 and 589 individuals in 2000. This means that lek counts should decrease
from 140 males in 1999 to approximately 132 males in 2000. This model can also be
applied to the entire Canadian population by combining Alberta lek counts with the most
recent lek counts in Saskatchewan.
3000
Low
High
Predicted Low
Predicted High
Number of Birds
2500
2000
1500
1000
500
0
1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 2030
Year
Figure 4.3. Actual Sage Grouse population in Alberta based on lek counts for 1968
through 1999 and predicted population from 2000 to 2030. Years when sampling
efforts consisted of less than eight leks surveyed are not included. Lines are
shown to illustrate trends for years with consecutive lek counts, and to show the
predicted population trend.
93
The model predicts that the Alberta population will fall below 300 individuals by
2004 (Fig. 4.3). Similarly, the Canadian population will fall below 300 individuals in
2013. By 2018 the Alberta population will decline to less than 100 individuals and the
Canadian population will be below 190 individuals. Based on 1999 productivity and
survival estimates, yearlings should represent approximately 49% of the 2000 spring
population. However, only 25% of the birds captured over the course of my study were
yearlings (Chapter 1). A lack of attendance by yearling male Sage Grouse at leks also
suggests that yearlings are under represented in the population (Figure 1.6). To improve
the predictive capabilities of my model, some parameters need to be refined, including
both adult and chick survival rates. If juvenile overwinter mortality is considered and all
other parameters remain constant, to have a yearling to adult ratio close to 25%, juvenile
overwinter survival would be approximately 40%. There currently are no data available
on juvenile overwinter survival. However, given that adult mortality is high, 40%
juvenile overwinter survival may be a reasonable estimate. If I reiterate the model using
40% overwinter juvenile survival, the Alberta population will decline to zero in 2017,
and the Canadian population will approach zero in 2019.
It has been suggested that Sage Grouse populations cycle (Patterson 1952, Rich
1985, Aldridge 1998, Braun 1998, Hyslop 1998), although these cycles tend to be
irregular, if they do exist (Braun 1998). Both the Alberta and Saskatchewan populations
appear to cycle (Fig. 1.3), however, inconsistencies in lek counts (Madsen 1995b,
Aldridge 1998) and the irregularity with which counts were performed, make it difficult
to accurately characterize these cycles.
94
Even though the cause(s) for the population decline in Canada are not definitively
known, I suggest it is likely the problem is related to poor chick survival and low
recruitment. However, reduced adult survival may confound the problem. It is possible
that a lack of genetic diversity due to such low population numbers could be adversely
affecting the population. It has been suggested (Franklin 1980, Lande 1988, Braun
1995), that to maintain genetic diversity, individual populations should consist of at least
500, and maybe even 5000 individuals. However, Sage Grouse have persisted in Alberta
for over 30 years with a population that was likely below 5000 individuals. These
estimates assume that random mating is occurring and that all individuals in the
population obtain mating opportunities. However, with a lek mating system, Sage
Grouse mating is not random. While most, if not all females breed, as few as 10% of
males in the population will obtain successful mating attempts (Anonymous 1997; C. E.
Braun, pers. commun.). This would mean that as few as 14 males would obtain mating
opportunities each year in Alberta (based on 140 males counted on leks in 1999).
At any given lek, one dominant male typically performs about 75% of the matings
at that lek, and one to three other males will obtain the majority of the other 25% of the
matings (Simon 1940, Scott 1944, Wiley 1973, Gibson 1996). Thus, with eight active
leks remaining in Alberta, approximately 24 males likely obtain all of the successful
mating opportunities. Assuming that 100% of all estimated 280 females mated, the
effective population size [Ne ≈ 4(G×E)/(G+E)] (Ewens et al. 1987) for Alberta would be
88 Sage Grouse. Similarly, based on 18 active leks and an estimated 542 females in
1999, the effective population size for the Canadian population would be approximately
196 individuals. Braun (1995) suggested that populations with less than 500 breeding
95
individuals in Colorado were at risk of extirpation. The Canadian population is far below
these suggested minimum levels and genetic diversity may be confounding the problem.
Bouzat et al. (1998) found that a population of Greater Prairie-chickens
(Tympanuchus cupido) in Illinois experienced a decrease in genetic diversity due to
recent geographic isolation. This decrease in genetic diversity was associated with a
reduction in population fitness, through reduced hatching and fertility rates. This
population was below 50 individuals, compared to larger populations with greater than
4,000 individuals that have not experienced decreases in genetic diversity or reductions in
population fitness.
With the Alberta Sage Grouse population estimated at between 420 and 622
individuals, and the Canadian population at between 873 and 1293 individuals, the
potential exists for a reduction in population fitness due to decreased genetic diversity.
Sage Grouse populations appear to cycle every 7-10 years (Fig. 4.2) (Patterson 1952,
Rich 1985, Aldridge 1998, Braun 1998, Hyslop 1998). Due to these cycles, populations
at low numbers may be more susceptible to extinction events, and genetic bottlenecks
could easily occur. Egg hatching success for Sage Grouse over the course of my study
was relatively high (92%, Table 4.1; Chapter 2), and suggests that genetic diversity may
not presently be a problem in the Canadian Sage Grouse population. However, low
genetic diversity may potentially affect population fitness in other ways, such as reducing
chick survival, or make the population particularly sensitive to stochastic events. Even
without considering these potential negative effects related to low genetic diversity, my
model predicts the population will approach zero over the next 20 to 30 years.
96
Without adequate habitat management, I predict that the Canadian Sage Grouse
population will continue to decline. The cause of the decline is likely linked to poor
chick survival. While survival estimates are slightly lower compared to other
populations, all other measures of productivity are comparable to similar populations and
do not appear to be linked to the population decline at this time. Habitat use by broods
indicates that variations in forb availability do not exist, that overall availability of forbs
is low. There appears to be a lack of moist wetland type habitats that provide succulent
food resources for Sage Grouse chicks, despite above average spring moisture in 1998
and 1999. Competition with cattle for already limited resources may compound the
problem, especially in dry years, when abundance of forbs may be even more limiting.
The Alberta and Saskatchewan populations may not be distinct, but movements
between southern populations in Montana are unlikely due to the lack of contiguous
habitat. Thus, given the lek mating system of Sage Grouse and the presently low
Canadian population numbers, a lack of genetic variability may confound the problem.
Management practices need to focus on the fact that given normal reproductive
effort and success by Sage Grouse in Canada, small fluctuations in chick survival can
have profound implications on population size. Efforts should be made to maintain, or
preferably, enhance suitable breeding and nesting habitat, while attempting to enhance
brood rearing habitat through the protection and creation of mesic habitats with high forb
availability.
97
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